I've only got a couple years of BJJ experience under my belt. I think what fullerene said is a pretty good analogy, though. It's just a different philosophy much like in standup some guys like to be aggressive and step right in the mix whereas others like to dance around the outside and counter punch.
In BJJ competition it's always about improving your position with the other fighter, so obviously when you're in their guard you want to escape that and try to get side mount or full mount. In MMA some guys don't want to be in any kind of JJ position and they have to worry about escaping and getting the fight back to their feet.
With some guys like Tim Sylvia if you're in his guard obviously he can still reach up and smack the shit out of you, so you'd probably want to go head low in that instance, too.
Just some thoughts
These guys are lightyears in experience beyond myself, though. So I can only guess at the reasons and I'm sure each fight it's a different reason