For me, its my lack of motivation to broaden my champion pool.
I very much enjoy OTP-ing offmeta picks, and the lack of flexibility sometimes puts me in a position where i counter pick myself.
For the past 3 seasons i have almost exclusively played, Nocturne top and Leona jungle.
I am able to get deep into Plat easily, but i cannot break into diamond doing this, and im aware its because im not playing champion select the way i should be to maximize my chances of winning.
I dont have bad games, but because i ignore the matchups there are many games where me doing well is irrelevant/and or i cant push my lead into a win.
But i do have fun, and Plat1 isnt all that bad. I guess.