costs. itx and mini itx and mini atx there's no room at the end of long cards if you can get the long cards to even fit, so when they design they design for the machines they already use which must have side mounting. using a 10 million dollar power connector machine for one card isn't very profitable to have to buy another for another type of card but using the one machine for every card can save you upwards of 80 million dollars.
i like the cables going out the rear also, makes for far easier cable management and better aesthetics.
The PCI adapter is something that I cannot believe we have not seen before. Its a solution to a problem that most people just accepted. having those cables exposed right in a case window never looked good no matter how much the cables were colour cordinated