I think HP had a big investment in Itanium and even sued Oracle to get them to keep supporting Itanium (they had contractually agreed to it, but Itanium was such a dead end...)
They since discontinued HP-UX, offloaded OpenVMS and ported NonStop to x86-64.
Non-Stop got ported early on because they needed some hardware and clients were locked down.
OpenVMS customers however balked about shifting to Itanium given that it was often slower than their existing Alpha boxes, leading to HP being forced to finish EV7 work and release another generation of Alpha chips (funnily enough, EV7 might have forked and lived on in Opteron/K8...)
From what I remember the only drama was that there was a missing pthread_ API implementation on the platform, might have been pthread_gettime or something.
Compared to the HP-UX on PA-RISC box I had access to at the time, the performance was amazing!