The reason why pre-selecting role lowered champion-diversity
Previously you could decide which role and champion you want to play based on the enemy champion picks. If you knew unusual picks that are strong against certain champions you could decide your role on the fly.
I'm for expample a jungle/support main but like to play Galio against Fizz, Brand, Swain or early AP-Nunu against Jax.
With the new system, were many people complain about beeing autofilled anyway this is no longer possible.