Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
When you work with a distribution you will see its warts and every distro has some if not many. If we have the choice (or decide we want to have a choice) we end up with the distribution that has the least annoying warts, we pick the one that sucks least.
When building your own distribution it's often probably not too smart to start at zero. There's been years of work in packaging and in testing certain ways of doing things, it's just smart to pick something to base your distribution on because you can spend the time developing what makes your distribution special instead of having to take care of the base system of kernel, glibc and whatnot.
The question which distribution you base your work on is often an emotional one: You've gotten attached to one distribution, you have put time and energy into learning it so you base your own distribution on what you know. But if you take a step back you might consider a few different distributions and often people end up with the same decision though I don't think that they always thought about it properly.
Many new distributions are based on Debian or nowadays sometimes Ubuntu (which again is based on Debian) and you have to wonder why. The more I personally work with Debian and Debian based distributions the clearer I see the big warts.