Debian is more bare bones then Ubuntu, that’s why. Ubuntu comes with a lot of packages already installed by default. In Debian you have to install a lot of that stuff manually. You might also have to edit some configs for example. It’s not that hard, but maybe a little too much for a beginner.
I upgraded Debian to 12 last night, which required manually updating the source.list for the apt repos for example. It’s been a while but I’m pretty sure Ubuntu gives you a UI for upgrades? Upgrading Debian was simple for a techie who’s played around in Linux already, but it could be more intimidating for a newbie.
The carriers refused to do it one their own so Google had to provide the servers themselves. Apple could do the same, but we all know they won’t and never will. If it wasn’t this excuse it would be another one.