The ppa repositories are an Ubuntu thing I've heard, there seem to be slightly more packages for Ubuntu-based distributions, and lots of things are just pre-packaged. Instead of having to apt-get a filemanager and everything, things are compiled by people that probably know more about it than me. Like with laptops, I'm pretty sure I could assemble one if manufacturers let us, but the real experts can probably do it better (with heat management and whatnot).

That said, Debian works perfectly fine for me on my phone. It runs lxde chrooted in userspace (Android is the booted OS), and I have no trouble using and operating Debian as far as things are available for ARM. Even compiling new things for ARM is usually no issue, though things like compiling Firefox are probably beyond what I would want to try on my phone :P