Earlier quoted context omitted.
Probably because they have to test the ROMs on their network in different regions, with different frequency bands, with roaming, etc. That work is tedious given all of the phones they support.
The iPhone doesn't suffer from this problem. It should be up to the phone manufacturer (HTC, Samsung, etc.) to test their product. The carrier can still, of course, test the new firmware all they like - they just shouldn't be able to interfere with its release.
Moreover, in some European countries it is normal to buy a phone and a subscription separately. In some countries bundling is even illegal. I have never had a carrier-branded phone since I switched to smartphones, including my Android excursion (Nexus 4, Moto X 2013, Moto G, Moto X 2014) and some Windows Phones that I played with (Lumia 710/920/640). They all worked on multiple carriers.
Everyone should stop perpetuating the carrier testing myth. For this we have the GSM standards and in many parts of the world a large number of phones are not carrier-branded.