Because a player who is not charismatic in real life should not be allowed to play a charismatic character, right?
Yeah, I used to be swayed by the argument. But no amount of charisma based skills is going to make an uncharismatic or socially awkward player less so.
All it seems to do in actual play, is muffle the more outgoing players while the less outgoing players are nudged into making a diplomacy roll.
Why? Well because certain classes in 3e became "talky" classes, and others were prevented from doing so. Rather than let shy or awkward players stay in their comfort zone (and maybe loosen up) they are instead pressured into becoming the face of the party because they have the right class or trained skills.
It is simply unecessary. There is no reason why one class or another should be better at roleplaying and diplomacy than another. Not even the bard, as 4e proved with the mystic and far-sighted oracle bard.
Why not instead let shy and awkward players blossom at their own pace, in their own way? Perhaps with the more socially adept and experienced players showing the way.