I'm thinking because anything Ten/Rose is going to fall into two categories: either it's AU, or it's historical (in that the story takes place pre-DD). While there are plenty of AU fics around, there seems to be a sort of block (be it mental or otherwise) in starting an AU fic.

And historical, you're sort of setting the characters up for the heartbreak that is DD, unless you plan on turning it AU anyway.

Neither of which explain the Nine/Rose thing, but that's a whole other story....