ever notice how the most popular champs are ones that have peel, mobility and damage built into their kit? no-one wants to rely on their teammates for this. nobody wants to wait for their team to initiate before they can do their thing. nobody wants to initiate the fight and then just stand in it soaking up damage. nobody wants to follow their carry around and keep them alive while they do all the fun stuff.
People want to be able to act on opportunites and fully capitalize on them without hoping their team recognizes their intentions. As long as riot keeps making versatile, flexible champs - even ones without specific strengths - they will always be more popular than champions with one purpose.