Correlation is not causation. That style is not popular for its virtues (expandability), but for its price. That's the long and short of it, especially since notebook sales dwarf minitower sales, and if you cut out the sub-$800 range, you're left with a very small number indeed.If you want an expandable Mac, you need to go for their hi-end workstation. And what is the most popular form-factor in desktop-computers at this moment? It's single-socket expandable minitower.
If manufacturers made a computer that had all the expandability of a DVD player with a RAM slot, but it was cheaper than the basic minitower configuration, it would be the most popular. You can confirm this easily based on the discard vs. upgrade rate of computers. People in general just buy new ones when a simple RAM or hard drive upgrade isn't enough. "System builders" are the niche market.