Imp said:This is simple.
Charisma is a measure of how kewl you are.
Magic is kewl.
Problem resolved!
Except that it doesn't explain why all spellcasters don't use Cha.
Personally, I think it would make more sense to switch divine and spontaneous arcane casters -- let the sorcerors use Wis and the clerics use Cha. Charisma is the capability to influence others; divine casters cast spells by petitioning others (the gods for clerics, natural spirits for druids) to intervene on their behalf. Sorcerors, however, are much like wizards, except for the distinction between intuitive sense and accomplished learning -- which is pretty much the distinction between wisdom and intelligence.