Because it's NOT anything, it's a Rude Gesture, that the monster finds offensive. Not something that really should be boostable by magic, but something that could reasonably be more effective, if used by someone with more force of personality.
~Rex
But isnt that what a charisma buff would do? Make you feel more confident, thus increasing your force of personality, thereby making your gesture more intimidating/offensive/whatever?
I guess that depends on how one would view the buff... I've always thought of Eagle's Splendor as making the recipient more attractive rather than more confident, hence my suggestion not to count in the buff.
The same applies to, say, the CHA buff from bathing in the tub in Tigan's Rest. It would make you more attractive since you're clean and don't smell bad, but it wouldn't necessarily make you more confident.
Basically, the way I see it:
Warding Gesture is presenting the holy symbol in front of the vampire and holding him at bay with your own force of personality/will.
Warding Gesture with CHA buff (Eagle's Splendor or whatever) is presenting the holy symbol in front of the vampire, holding him at bay with your own force of personality/will and looking good while doing it. And I highly doubt the vampire cares whether you look good or not.
Don't get me wrong, I'd benefit alot from being able to get my fighter's CHA buffed and using it with Warding Gesture but not counting the buffs seems a rather fair way to balance Warding Gesture against Turn Undead.