This was a question I found myself asking myself today. It just came to me.
Well I went on line and found this answer. It seems to be coming from a cathlic site. There will be link at the bottom.
Mostly it's it got started so far back, they can only guess as to why, but it was adopted easy enough and now it's a given thing that is also some kind of status thing.
For the list of how it could have come about. check the link. :) Oh, the hair cut actually has a name. Tonsure.