1. CPAN modules that will not work with Perl 6/Parrot.
2. It seems that everyone is writing their very own VM these days (ex: java, .net, mono, parrot), which will (of course) run faster, better, etc than everyone elses. There is hardly a practical technical incentive (platform dependency excluded) to choose one VM over the other. Parrot will be just one in the crowd.
3. Will not appeal to VM zealots because it is too late in the game.
4. Will not appeal to perl programmers because it will probably be bigger, require more typing, run slower than perl 5.
5. They are not really thinking about making extending/embedding easier than XS.

Then again ... I might be completly wrong.