Itanium was compatible with x86. In fact, it booted into x86 mode. Merced, the first implementation had a part of the chip called the IVE, Intel Value Engine, that implemented x86 very slowly.

You would boot in x86 mode and run some code to switch to ia64 mode.

HP saw the end of the road for their solo efforts on PA-RISC and Intel eyed the higher end market against SPARC, MIPS, POWER, and Alpha (hehe. all those caps) so they banded together to tackle the higher end.

But as AMD proved, you could win by scaling up instead of dropping an all-new architecture.

* worked at HP during the HP-Intel Highly Confidential project.