It's his utility.
I'd go a step further and say he doesn't have the kit of an assassin, he's more fighter that likes diving on people's faces and locking them down with CC like:
Pretty much everything in Ekko's kit benefits heavily from building tanky just with how it works.
. HP shields and self-healing? 100 HP is the equivalent of 200 HP if you have 50% damage reduction.
. Slows and stuns? Great with tanky CDR builds where you can effectively double the defense/offense of your allies by keeping enemies in range longer and preventing them from striking your buddies.
. Diving onto the enemy team with mega-AoE? benefits from being tanky enough to initiate a fight.
. 3rd hit AA bonus? Benefits from being beefy or having a lot of range, and doesn't benefit at all from being mobile melee squish. If you're reliant on speed to survive: you want to move around as much as possible, where chain auto-attacking really slows and telegraphs your movement.
The only part of Ekko's kit that really benefits from a glass-cannon, high-risk, high-reward playstyle is his capacity to jump onto 1 single opponent with Phase Dive. If you're a melee glass-cannon you want to have a lot of speed to survive through re-positioning and dodging incoming fire, as well as really fast and responsive skills that don't lock you in place or lock you out of evading bad situations by having long casting times, slow projectile speeds, or requiring you to be sticking and utalize a string of auto-attacks.
Magic damage and QWER burst also require less offensive itemization than Physical damage auto-attack focus.
At level 18 average QWER burst hits ~1000 and requires a massive 450 AP to simply double to 2000, which doesn't really punish dipping into tank stats. Reason why a lot of mages can run items like Zhonya's, RoA, Rylie's, and Abyssal without giving up much.
Physical AA's only go up to ~110 every 1 second @18, where itemization offers the potential to octuple that damage with a committed build: and that damage is pretty much non-existance without itemizing for it.
Also, armor is generally 35%~50% higher on all champions and monsters due to bases and often itemization: giving magic-damage champions essentially a free Last Whisper (from past seasons) in free penetrations for the entire game. With that and bases, magic burst tends to offer pretty relevant damage throughout an entire game without any items backing it up.
To make matters worse: AP itemization is struggling at the moment and AP assassin itemization is largely non-existant, where tanky DPS fighter itemization is pretty amazing at the moment.
Where Ekko's entire kit already pushes him a bit into surviving twice as long to do twice as much damage rather than building damage to deal twice as much damage, current defensive itemization is offering more than double survival and AP itemization is offering less than double damage.
It's bad enough most assassins released into LoL wind up being tanky DPS fighters, but Ekko really isn't even an assassin in the first place. He's more a fighter/tank that has the option to assassinate things like AP Malphite.
Riot plz.
are all good examples of this because they have little CC to go along with their high damage kits. Some have a little more than others but don't even come close to the degree that Ekko has.
forgot about her! She has hard CC on a pretty low CD
is allowed
