Not enough people on the bandwagon, competition from Ruby/Python/JVM languages on the high end and Perl 5 on the low end, and last but not least the fact you can't use libraries written for Perl 5 with Perl 6.

I wrote a medium-sized project in Perl 5 two years ago. It is my day job right now to work on it. I have very little interest in rewriting it from scratch in any language, but if I did, it would probably be either Ruby or something on the JVM.