I mean you look at him and can't help but notice that Rammus is almost never played in any elo, I mean what is it about him that results in so little use of him?
He's got mobility, CC, built in thornmail that stacks with ACTUAL thornmail, a passive that just lets him go deal LOTS of damage with just going pure tank, and an ult that deals AoE damage to minions, champs and towers, so what makes motivates people to never play him?
Tworie5 votesBecause Vayne and Master Yi exist. Rammus should be good against auto attackers, but these two have true damage in their kits that make stacking armor useless.
Glîtchy1 votesBecause Vayne and Master Yi exist. Rammus should be good against auto attackers, but these two have true damage in their kits that make stacking armor useless.
Thornmail and his ability backlash the true damage, also that's why you build 
and 
oSEXYPLATYPUSo1 votesbut with vaynes W and bork she does more damage to you the more you stack health...
and most people wont take that into consideration.. and what would actually make him more a threat to her would be magic pen to go with cinderhauk's aoe magic damage, and his build in "thornmail" because with more magic pen she takes more damage as she attacks you... and taunt her (get landrys) which would help with magic pen, ap , hp, and dot damage which makes his abilities more dangerous. however for that being said rammus isn't played more because its too easy to beat him and invade his jungle early and he really only does well vs certain situational team comps.
hhaavviikk1 votesThornmail and his ability backlash the true damage, also that's why you build
I've played as rammus vs 5 AD team so I could go full armor, still lost to vayne and yi because true damage, it sounds nice in theory, doesn't actually work out
Marshbouy3 votesHe's super boring and snowballs backwards if you get behind.
Troy2426211 votesI don't play tanks in general because they have an inherent team reliance that can screw your games if you don't literally run the map down before mid game, where they have the capacity to throw the lead you gave them.
Spoofghoul1 votesbecause % pen exists and the over abundance of lifesteal heals and shields make his niche ever more useless
Sûros1 votesEasy answer for this.
Riot decided LoL was a game about melting things ASAP. AD champs now have access to items that will half your armor, destroy a percentage of your total health, and steal/destroy health faster if the target has more.
This has lead to even the most powerful tanks falling apart in an average of 3 seconds by endgame where they used to wield the greatest power. What do they have access to? Cutting their attack speed by 15%, dealing retaliation damage that nobody cares about anymore, and shields that might block a quarter of a hit.
Try taking Rammus into a fight with 400 armor and see if the ADC cares that you take 3 seconds to kill instead of 1. Game is about burst now and not strategy.
A Basic Island1 voteshe's got "mobility" but doesn't have mobility
Illâoi Evê Siôn1 votesBlack Cleaver and Last Whisper are too powerful at the moment, plus he struggles against the Casters and Enhanced Basic Attackers of the enemy team. In the Tank Update Black Cleaver will be nerfed and new defense items will be introduced. Maybe he'll become more popular then? However, Riot has always nerfed Rammus when he was a popular pick in any meta. Hopefully he'll get more popular, but not popular enough for that to happen.