The other reason why competitive diversity sucks: Match-ups
Short n' sweet: The game's way too forgiving about champion match-ups to the point where the game's always in a state of "These few champs are better than EVERYONE ELSE in their lane".
Longer post: League of Legends is built upon a foundation largely supported by RPG concepts and mechanics, right? Damage types, classes and sub-classes, spells and passive abilities, level ups...all that jazz. However, Riot's balancing philosophy is akin to fighting games, rather than RPGs, where match-ups are as close to 50/50 as possible with the main defining difference between characters being the logistics of how you fight. Why is this bad for competitive diversity? Well, that's what this long post is for.
Pro play is a showcase of LoL being played the way it was meant to be played at a level where the best the game has to offer can be showcased. Basically, it's the endpoint of Riot's goals when they started Season 6 with DQ that they recently gave up on. Now, everybody knows that the meta for pro play is very different from standard play because standard play lacks coordination and consistent teammates. This leads to things like Shaco working in ranked games...because it's hard to accurately coordinate with your fellow players WITHOUT COMMS IN THE GAME.
With this foundation laid out, when the game's being played the way it's meant to be played (with team work in a team game), champ diversity is pretty bad. Like, I understand that pro scenes tend to have restricted rosters, but the meta diversity for this World's tourney is like a third of a 130+ champ roster. And that's because Riot's taken a "Every match-up is as close to 50/50 as possible" route rather than a "Each champ has clear cut strengths and weaknesses, and that's fine" route, which is great for keeping the low elos (which is the majority of the player base) happy, but sucks for the pro scene spectator experience. Because of the balancing philosophy we currently have, where champs are kinda "soft countered" at best, pros aren't punished for just picking meta giants all the time, and then those giants get nerfed, and then people that aren't in the pros get nerfed for something that hardly applies to them, even though it's a real and entirely possible issue. The tournament itself is less about making unique strategies against new opponents as the draft and match goes along more than it's about 2 teams doing 2 variants of the same thing, based on what's strongest to do, who's the best at doing them, and who got which champ first. It's boring to see it, and the game's meta doesn't "flow" more than 
"Riot forcefully shoves it around based on if they think we've had enough of it already" because it's sucks to be in a situation where "because I picked X, they punished with Y". However, now it's like "Why the fuck didn't we ban Nidalee while we're in Diamond again?"
Let's compare this to Pokemon for a sec, like I'd predictably do (I was going to compare it to another game, but this example is clearer since Pokemon's an easy to understand game). In Pokemon, you have tanks, you have sweepers, you have tanky bruisers, and you have offensive/defensive support pokemon. Everyone has their role, but the difference between them goes beyond "I do my thing differently than you" like a fighting game would. It's more like "I do my thing differently, I'm super strong against these guys, and I'm super helpless against those guys". Like, let's say you have my personal offensive support Raichu I bred last year, right? Lil guy's mighty squishy and doesn't do all that much damage, but he was integral to my team because he could slow 
multiple enemy pokemon at once (Electroweb), 
straight up cancel attacks (Fake Out), and 
did a "decent" amount of damage when required (Thunderbolt) (last move was Protect...doesn't really compare to this game too well). Still, he's an Electric-type that only has Electric-type moves and a move that can only be used once every time he's thrown out, so in a competitive setting, if my lil guy's walled by, say, Marowak, a pokemon that's immune to Electric-type moves...what happens? Well, put bluntly, I'm royally fucked unless someone else could quickly get rid of that thing in 1-2 turns, hence why I had a Milotic too. If Milotic went down before Raichu? I'm even more royally fucked.
Still, the type of thing that pushes diversity in Pokemon is the notion that you can't run everything over with one type of pokemon. You have to build a team that'll balance out the weaknesses of its members as much as possible with its strengths, and yes, some pokemon are straight HELPLESS against others (either by the enemy doubling up on resistances, your guy doubling up on weaknesses, or the enemy being immune to your attacks). It sounds harsh, but that's just how RPGs work. They rely on a system of checks and balances based on who's around and what they have, and the reason why is so that you don't pick "the best one" and bend the game over a barrel.
Now, if you're acquainted with Pokemon, there's another side to this coin...basically that the strongest picks actually DO bend the game over a barrel. Mega Kanga, Mega Gengar, Mega Mence, legendaries...they basically take the meta of the game whenever they're in play and wrap it around themselves by virtue of having insane amounts of damage, type coverage, being way insanely tanky, etc. and so forth. This is the other point: When you have a game that has clearly dominant options, that option takes over the game. That isn't just an RPG thing more than it's a video games thing, and that's why Pokemon's been steadily having issues with team diversity for the past two years when they allowed more and more legendaries into the VGC format. However, in 2014, there was a freaking Pachirisu on the winning team of their biggest tournament. A lowly supportive pokemon you can find in like, the earlier stages of the games...basically an Electric-type Butterfree...being a major factor in winning Pokemon's biggest tournament that year. Also led to some really cool fan art.
The point of comparing LoL to that type of game is that, well, LoL's foundation is made from the stuff of that type of game, but the game's diversity plummets when played in a more organized format because Riot's attempting to appease the feelings of lower elo players. When you scrunch up the match-ups between champs to be so close between themselves in their lanes, it becomes less about picking the right champ for the job and more about picking the best champ, period, and even though disorganized, clown fiesta-infested yolo-queue doesn't reflect that, organized play does and will continue to because they're tapping into what LoL actually is. If we want to see more diversity in pro play, the game needs to be balanced less like fighting games, which are more prone to top tier picks rising and squashing everyone else unless you're a godly low-tier hero due to the fact that counters aren't usually hard counters, and more like an RPG, where each champ has an intentionally clear cut space where they're strong or weak with intentionally made strong/weak match-ups. As an added bonus, it'll give Riot more creative freedom when designing champs, because it'll be possible to create a niche for the purpose of countering another niche/champion sub-class. It's something Riot's dabbled in a 
little 
bit 
, but it could be taken further.
Side note: This isn't me saying "All assassins should be able to .02 second murder my marksman without ult". This is saying that "Some assassins should be especially suited for dealing with burst mages, or other assassins, or artillery mages, or marksmen, or this assassin works especially well with this enchanter, or that disruptor, or that vanguard, in their general sphere of being assassins", then this concept should be spread around throughout the game so that "This X should be especially strong with Y or against Z". Just sort of polarize and emphasize match-ups and composition synergy so the game naturally promotes champion and strategic diversity, rather than the rather provocative suggestion of enforcing it in pro play by locking out picks.
Champion kits post here: http://boards.na.leagueoflegends.com/en/c/gameplay-balance/ALEVfMb4-my-problem-with-whining-about-competitive-diversity
