It's a combination of historical, technical and liability reasons. Even though wireless is built on standards (GSM, etc) implementations by vendors don't always conform, have defects, or need additional configuration to make it work well on a particular network comprised from particular base equipment. Does Qualcomm test every one of their their modems with every firmware version on every base station from every vendor? Does MediaTek? They don't and so many issues only show up once you encounter those combinations at the carrier end. When a carrier sells a phone with service on their network, they want to be damn sure it works or they'll be getting a volume of angry calls or worse (think implications of failed 911 calls). In order to ensure that doesn't happen, carriers test every device and every firmware version for it on their network. A lot of vendor hardware/software defects are found precisely during this stage. That's because these days the software/firmware is just as important for the proper functioning of the device-network combination. Therefore carriers want to keep tight control over device/software/firmware combos and only sell what they've tested and certified as working correctly. Ultimately carriers hold the keys to their network and they can block any device/group of devices that might cause problems for the network, others subscribers or expose them to liability. This is why updates that can affect any of the above are gated by carriers. This used to be extraordinarily strict prior to 2010. We made specific device software versions for Verizon, AT&T, etc. Remember when every phone had a carrier logo on it? That's when that was. After 2010, manufacturers and software vendors gradually began decoupling the carrier-related pieces like modem firmware and services, telephony, emergency, etc from the higher level software like calculator apps and through.. arduous lobbying and discussions with carriers vendors proved to carriers that they don't need to vet every little thing that changes in a phone from one version to the next. And so gradually we had more and more things not having to pass carrier testing. That said, OS updates which change the aforementioned critical components are still gated by carrier testing and Google can't do shit to get past them. They have no leverage as they wouldn't be liable if something goes horribly wrong.

When I say carriers, think large North American carriers and large European ones. Verizon, AT&T, T-Mobile, etc. Small carriers that don't have much resources don't do as much testing and rely more on the diligence of the vendors on the phone and base station ends to have things work well. They also benefit from fixes of defects found during carrier testing conducted by the big ones.

Source: Am a software guy that used to work at BlackBerry during the BBOS, BB10 and Android days.