Money, time, and experience.
All of these things have changed since over the past year or so, and there's been a lot of internal discussion about moving to Debian. In fact, it's almost sure to happen for Isis+1 (Isis is the name of our next release).
Basically, setting up our own repo took really good packagers (we have them now, and you can also get pretty much all of our software from Gentoo and Arch), running our own repo required a solid and cost-effective server (we have this now), and forking let us inherit a LOT of stuff we didn't have the resources to customize back then.
As we grow, it seems only natural that we base less on Ubuntu. But a lot of us used to be Ubuntu developers/enthusiasts, and a bunch of us have good ties to the Ubuntu ecosystem (a number of us met up at UDS-O), etc.
So, tl;dr, it will happen soon enough :)