I did some searching, and as far as I can tell, there has been no reason given for the removal of collision detection specifically between players or players and mobs. If anyone can find one, I'd be glad to see it.
However, a less specific statement can be found in the table of update plans as seen in Letters from the Producer, such as III and X; the statement occurs under the plan of environment collision detection, but the statement itself seems more general, referencing "collision detection" of no certain kind. There is a separate item in the table for character collision, but it is left unexplained.
"Risk analysis of collision detection adjustments to
improve movement response"
Interpret that however you like; to me it says that collision detection adjustments are being made in order to improve movement response, where
response is used in a technical capacity to refer to the concepts of
response time and
responsiveness (
http://en.wikipedia.org/wiki/Responsiveness).
I'm not saying I'm right; but I'm not pulling **** out of my *** either.