Itanium was a trojan horse to get their rivals to give up on MIPS/Alpha/UltraSparc development. As soon as those CPUs lost all traction, Intel dumped Itanium as well. Intel was never serious about the Itanium. Considering how performance sensitive it ought to be, Intel chose to build Itaniums on older manufacturing processes. The fact that Itanium performance was hopeless should not have been a surprise.

Itanium was EPIC, pretty much the opposite of RISC. Intel bet on compilers being good enough to tell the CPU which branches it could execute in parallel, and lost that bet.

Itanium was a RISC-y endeavour by Intel and it had it's problems but you can't really say it died because of x86_64. Microsoft adopted it and dropped IA-64 like a hot potato. Intel dropped Yamhill and other projects that attempted to bring IA-64 to the desktop as soon as they realized MS wanted x86-64. Performance didn't factor in because AMD would have never had the chance to provide the world all the CPUs it wanted (supply constraints)