I don't know the answer to the question, but I've noticed the same phenomena occur.
I think a part of is something we read about in psychology class: the person who talks the most in a group naturally becomes the leader of the group. But then again, the person who listens the best can achieve the most power because they will have the best understanding of the group dynamics...although that is only if they choose to act upon it.
Another thing is that, I personally find very shy/quiet people to be scary. It's almost like they feel they are too good to talk to you, and when you have someone shy/quiet in a group it is a little awkward so you try and avoid them. I used to be the shy/quiet one a long time ago, and the reason I no longer am is because:
Haven't you heard the saying, be careful of the quiet ones? (or something like that) ...think of murderers, pedophiles, and other psychos...they're usually not loud and obnoxious, but very quiet, to themselves, and very creepy.