I'm not an expert, but my understanding is that HURD made some uniquely poor choices that make it difficult to work with in ways that other microcernels don't necessarily suffer from.
http://walfield.org/papers/200707-walfield-critique-of-the-G...
One step forward two steps back...and no actual progress, that kills a project who is in competition with something like linux (who is obviously not a perfect design but just pushes forward)