Because it'd look like crap on the front, probably. The back is already crappy black plastic. Also, the logic board is probably around there so that avoids having to route some kind of cable from front to back in a cramped enclosure.

That was my first "wtf" reaction. Although they don't show it in the photos, the back of a mac mini isn't exactly a fun place to have to poke around all the time. Cables etc. Especially because (at least in older models) the cables tended to get loose after a year or so and could be easily knocked out. I wouldn't want to be reaching around there to pop the card in/out. It also limits the places you can mount the computer relative to where you're using it from. Seems stupid. They should've left it out. However, I guess it provides a good utility for doing occasional tech support (boot from SD etc).

This is where aesthetics fails over functionality in a big way.