Demand. Porting from PA-RISC to Itanium wasn't pain free for end users. Porting to Linux/x64 isn't likely much more effort than some HPUX/x64 that doesn't exist yet.

If you are going to x86_64 you might as well run Linux these day. The only people who might still need HPUX are people who have binary software packages that only run on HPUX, and they obviously won't run on x86_64.

Exactly. Whereas porting away from OpenVMS is a lot of effort, so it's worth the effort for VMS Software to port OpenVMS to x86_64