I don't have a theory. I know with a certainty why. Because low margins on low sales makes for poor ROI. Mac desktop sales, including the mini are insignificant compared to those of the notebooks and iMacs. They have been this way for over 6 years and they're only getting more insignificant each year.

The iMac is what started Apple's reversal of fortune over ten years ago and what prompted other manufacturers to start copying them.

The consumers have spoken and Apple has responded. If Apple had seen trends going in a different direction, they would have adapted accordingly. Unlike other product segments where Apple has managed to create its own category, the desktop PC market would, IMO, prove a lot more difficult. I believe the bottom line is that regardless of tricks or treats, the market just isn't interested in computers with the capability to be upgraded.

You can see for yourself the lack internal card products compared to external equivalents. Personally speaking, I don't know anyone (and have never known) who is not a computer geek who has ever purchased an add-in card or CPU upgrade on their own.

Back to the Mac Pro specifically, the case alone is probably worth some $500-$700