Because AGP is an extension of the PCI spec, and I believe PCI was 'flipped' from EISA at inception to help prevent people from sticking cards in the wrong slots.
And, since according to spec (and precedence) an expansion card is allotted the ~0.5" corresponding to the back bracket, major components are not placed on the 'back' of cards.
The only expansion cards I've ever seen with anything larger than SMT components on the backside are video cards, and that is still limited to memory(obviously). As another note to those that may be relatively new to the hobby: I didn't even see memory mounted to the backside of video cards until the 8MB cards hit the shelves around 1997.
Up until recently, stock cooling on cards did not need to be terribly fancy, and did not need to extend beyond that area (most of the stuff I have packed away doesn't even have a heatsink...).
Sure they run hot, but those temperatures are within design spec for those parts, and it's not a big deal for a large percentage of users that run stock speeds, and even a large percentage of those who run at higher than stock speeds.
Also, if you use a WTX case (or do some slight work to an ATX case to run it upside down) your PCI/AGP cards will end up being 'right-side-up'.
