6 of the 10 most played champions in diamond are adcs

The pool of viable ADCs is relatively small compared to basically every other role. Your options as an adc are basically: of those, and are pretty terrible at the moment and are played in other roles leaving you with a pool of basically 12 champions to choose from. Of those 12, you have which are objectively better than the rest in the current meta, or who have always seen a lot of play.

ADC's have the highest carry rate.

1. They have consistently high dmg
2. Unlike mages, and assassins, missing abilities doesn't cost their lives
3. They can kite forever