Because distros having multiple different distros was a bloody stupid idea to begin with. Fragmentation is a Bad Thing. Fragmentation in the UNIX market is what allowed the rise of Windows NT. It makes software development harder and it makes the deployment harder. A lot of documentation is useless, because it's written for a distro other than the one you are using. Tons of effort are wasted because different developers package the same packages over and over and over for different distros. And this also leads to stupid bugs like the debian openssl disaster or the recent bug in OpenSuse's screen locking thingy. Not to mention the fact that having fewer distros would make it a lot easier for users to pick one (Just watch the talk "The Paradox of Choice" by Barry Schwartz, you can find it via a search engine of your choice (no pun intended)).
My personal belief is that eliminating 99.9% of all distros would increase linux adoption massively. And while we're at it, we might as well kill gtk+ in order to give developers a uniform platform to the develop for (Qt, that is). Sometimes less really is more.