1) All non-combat skills in the game use Wisdom.
2) I wanted to make Charisma itself more versatile in more situations.
3) There's sometimes a fine line between the conversation skills, Bluff and Diplomacy especially.
4) Finally, I wanted players to read, interpret, and make decisions about conversation choices because of the language itself. Using Charisma, which is pretty broad conceptually ("compelling attractiveness or charm"), allows them to do this with some mechanical element still involved in success likelihood.
But that wasn't enough. While there are certainly payoffs to having high Charisma in terms of narrative, as well as a few scattered mechanical benefits, the benefits aren't nearly as consistent and predictable as what's conferred by Strength. I'll talk tomorrow about what else Charisma does.