1. Google had to give in to carriers *a little bit* so that a Nexus could get some features - like WiFi calling - it wouldn't get otherwise.
2. To get the Nexus line carried in stores. That way you can get it in your hands and try it a little before buying it.
3. Getting the Nexus line in stores means carrier financing, like T-Mobile's interest-free deal, makes a Nexus affordable for more people.
4. 1+2+3 = More features, hands-on trial, financing and better sales. It's still a Nexus, still runs vanilla Android.
All that, but I might get the next update a little slower. To me, that's an excellent deal.
Android since v1.0. Linux user since 2001.