Speaking personally and professionally, I'll say that ergonomics play a huge roll(no pun intended). You get to a point when miniaturizing things that they get too small to effectively be able to interact with them. I live in the TV world and if you look at a one piece Betacam from the 90's with it's tape transport in the body vs. a solid state camera today(with 20 years of technological advancements), the bodies are still pretty much the same size, same weight and same layout of key switches and buttons(even among different manufacturers). If I go in my office and pick up my first Sony Betacam from 1997 and then pick up my Panasonic P2 Varicam from today, the power switch, black balance/white balance switch, filter wheel, record button, white balance memory switch, etc. are all in the same places. The cameras sit on my shoulder and balance similarly. My point is, especially in the professional world, the way something feels and the way you are able to interact with it are more important than "lets make it small just because we can".
The point that I think some people are missing is that you can operate a DSLR "without looking" - i.e. while the viewfinder is up against your eye, you can change most or all of the important settings - shutter speed, aperture, controlling the AF system (not only focusing, but also selecting things like the AF points to use), ISO etc. all without lowering the camera to look at it. - Smaller form factors sacrifice that ability.