Why we cant choose Karthus?

Because his ult does 550 damage before counting his ap, and it's not fair for the enemy to be able to do 2750 damage to everyone.

Hes permabanned. I want to know why though.

Is he permabanned now?

That is really silly lol, he wasn't when One for All first came out. I guess having the ability to ult all at once isn't the most fair but it was still super easy to counter.