Control, $$$, efficiency, etc.
Think of it from a business standpoint.
This is it, really.
It doesn't cost them money for a P2P solution - or rather, it doesn't cost them anywhere near as much... it doesn't even really cost the user, as long as they're not paying a monthly fee to game, and would have an internet connection anyway. What the devs are doing is making the ISPs pick up the bill.
And as many ISPs in the UK (in particular) are
ultra hostile toward P2P or anything that increases load on their networks beyond basic web and e-mail - they even complain bitterly about the cost of streaming video sites like YouTube!
I know for a fact that a couple of UK ISPs now block Steam completely during what they call 'peak' hours (ie: between about 7am and 2am, so there is a five hour window you can game on Steam with) so I can't wait until more start doing it, and there is an enormous backlash against P2P type gaming. Bet it'll happen in the US pretty soon, too.
Imagine what would happen if an ISP blocked XBox Live?