The distros certainly have a lot in common, but each has its quirks and idioms, and once you learn them, it's annoying to have to bother learning another. Like, once I've memorized how to do a lot of things with RPM and Yum, it becomes "friction" to suggest that I learn to do the same things with apt.
I don't have any problem with Ubuntu, and I think it's fine for new users. I stick with Fedora really for two reasons:
1. The aforementioned familiarity.
and
2. They are more ideologically committed to software freedom, in terms of not shipping things like mp3 support, where you have patent encumbrance issues. I have been trying to gradually shift my life to a place where I don't need that stuff at all, or where I do, I use OSS / free software to interact with it. I mean, I'm pragmatic enough to use VLC to watch DVD's, but I don't go out of my way to use any content that isn't in a free format, etc.
Sure, there are plenty of differences, but these are mostly the equivalent of vim vs emacs - if you're good enough you can make either work. They're mostly just matters of personal preference.
Though many users don't have the benefit of your level of experience. And, there should always be room for new users benefiting from Linux. So Ubuntu's tended to focus on being a Linux that works "by default" on the desktop and make sensible choices for developing in the Cloud.