This is just blind speculation (I am not apple [power] user, nor have I owned the new mac pro):

Maybe (sacrificing customizability[?] for portability) + (somewhat superficial/look change maybe for change's sake) + (lack of (water cooling, cd/dvd/bluray%, power surge supply/protector thingy%, raid%)) != professional market?

% = if you want these, there goes some of the portability/simplicity. (unless apple monitor usually has cd drive?)

I might be off the mark, feedback welcome. Maybe this is enough for most creative professionals? Video/3d/gaming/some programming/some engineering might be somewhat infinite in appetite for processing power and all, I would think.