There were many reasons why it failed but I think the combination of these two was more synergistic than it seems. Itanium’s designers made some very dubious assumptions about the kind of code they needed to run (businesses buy branchy code, not hand-tuned SPEC benchmarks) but then the systems cost considerably more than the earlier predictions and almost everything was much slower than predicted because few developers had access to a system to tune it and the best compilers were not free.

The architecture was probably a fatal mistake on its own (even if Intel had disbanded its x86 line, the “older” RISC processors outperformed, often substantially) but ensuring that almost nobody saw anything close to what performance it could deliver definitely made it fatal. If Intel hadn’t been greedy trying to charge for compilers, the gap wouldn’t have been nearly as bad.