Charisma, which previously measured a person's force-of-will, was an influencial factor on several unused skills and was pretty useless in general. When Priests and Necromancers come out, this ability score was going to be used to help control and turn undead, but other than that, there was no good use for it, so it is being completely removed. We will have a faith or devotion system for Priests and Necromancers which is seperate from ability scores (which shouldn't be directly-trainable anyhow).
From a Dungeon Master's controlled perspective, it's very easy to use Charisma quickly to identify personality and presence, but for a computer system that can track many activities but is less interactive than a live human being, a seperate system is the way to go.