High skill cap champions are popular because...

...they have high skill caps. I mean people constantly talk about their play rates, and how they are played too much. But the fact is they are fun and have growth potential. How many people would rather spend their time mastering Garen over Riven or Ashe over Vayne? I love Ashe by the way. That is beside the point, more people are bound to play more often things they can grow with and that have somewhere to go with. They are less likely to invest in things that have no growth potential.

What is more, they are fun and offer a challenge. Pulling off a successful play on a difficult champion, is going to be much more pleasing to the average person than pulling off a successful play on a point and click champion.

They also have options, you can choose to do more than one linear action.

Now! I am not saying they are better designs, well... they might be. But just don't get the wrong idea, I main Shyvana, very linear. I also enjoy Ashe and Soraka. But I know for a fact that the champions I play have very limited growth in terms of skill cap, low options in game and have more linear gameplay.

So understand, just because champions with more mechanics, or even who are just flashy like Ahri, are played a lot. That doesn't make them OP. No I am not justifying any particular case. But some champions with high skill caps are OP, some with low skill caps are OP. But more people are GOING to choose high skill cap, fun, flashy champions who are viable, and invest more time into them. Because it has more inherent value for fun and growth.