It'd be kinda cool if a lot of people got their passwords stolen too
vBulletin has horrible HTML sanity checks. By horrible, I mean it actually doesn't have any. People could use Javascript to redirect people to cookie-hijackers and all sorts of stuff that could compromise the board's security, which is why it is always disabled.
Besides, what functionality does HTML provide that isn't already included in our custom vB Code? Apart from tables, I can't think of anything else worth having...