I have to admit that the primary reason was at first mostly pragmatic, Ubuntu seemed like less work than Debian. (I come from a background of Solaris, HP-UX, Slackware, Redhat/Fedora.) Most of the Debian users I have met in real life seem to have different computing priorities than me. I am not trying to be critical of Redhat or Debian.
I put Ubuntu on my son's computer, then my notebook, then transitioned a couple servers. It was a sometimes painful experience. (e.g. I realize many people would call me odd, and for a number of reasons, ;-) but I really prefer sendmail, and I ran into Ubuntu specific bugs in sendmail, and then others in postfix after I gave up on fighting the sendmail battle during my first server trials with early versions of Ubuntu.)
With all that said, I would not consider Debian now because I want a commercial company to do well in the Linux business. Actually, I want many to do well, and Redhat is one of my favorite companies - I really do not wish to criticize Redhat, it's employees, or Fedora. Their products just do not quite fit my needs right now. I still recommend RHEL, and have helped other companies deploy it. I do not think Fedora is appropriate for production servers though, unless the admins have a lot more time and energy than I do.