It's full of good ideas, but also full of cruft and years of technical debt. It doesn't have a JIT compiler, (nearly) no async IO, threading support is pretty new and not battle-tested nor documented very well.
Actually, that's part of the issue. Most of Parrot's semantics are exactly those of Perl 6 (right down to the opcodes for Boolean testing), and those don't get along well with most languages.
The fact that Rakudo on the JVM can take advantage of the JIT to optimize simple math in tight loops fails to impress me, but I suppose it's one more half-implemented feature you can tick off the back of the box, so hooray.
Because it's associated with Perl 6 which is commenly believed to be vaporware (yes, even if there have been releases). And because they want to make it so perfect that people can't remember why they used to be excited about it.
