Ubuntu was my first desktop OS when I started playing around with Linux (somewhere around Ubuntu 6), and then first server OS - I think mostly due to the Slicehost guides that were available at the time. Moved to Debian when I started tinkering with low end stuff just because of how tiny a Debian 6 minimal install was.
People stick to what’s familiar, and if you start with something and learn it well you’re less likely to jump over to something else. I remember moving to Mac OS in the mid-00s (just after the move to Intel) and it was quite the learning curve coming from the Windows world.
I may invest some time in learning and getting familiar with CentOS, but I just haven’t had a need to up until this point. Debian/Ubuntu have been more than stable enough for me, so the stability claims don’t sway me very much. Also considering how much I ‘cheat’ by using automated installers on personal boxes (EasyEngine, Atomic-Toolkit, etc) - almost all of those are built for Deb/Ubuntu primarily and seem to be most stable there.