I'm definitely more familiar with the Android side of things than with iOS but it really has more to do with the OEM. Sure, if you buy a carrier branded device

I'm definitely more familiar with the Android side of things than with iOS but it really has more to do with the OEM. Sure, if you buy a carrier branded device then they will stick their approval process after the OS release and OEM porting but in general, it's more a firmware/driver issue a lot of the time. Apple's strength is that they provide the hardware and the software so they are responsible for building any new version of iOS with applicable drivers and firmware for each device. Core apps are one thing but if you want to update to the latest version of Android, you're mostly out of luck without the OEM's involvement. Third parties regularly work on building current versions for all manner of devices as soon as Google publishes the source but without driver source from the maker of the actual chips and other hardware, they're left cobbling together unoptimized drivers for important things like the cellular radio or the camera or reverse engineering their own. If it was just a matter of carrier interference, there would be a "vanilla" build available for every device shortly after release.