As well as the other answers here, an additional reason was that during the time from announcement to shipping silicon the performance of x86 had improved such that the Itanium chips were lackluster performers in comparison to a top end x86 being produced by Intel at the time Itanium shipped silicon.

Couple lackluster performance compared to x86 with a significantly higher price than the equivalent performance x86 CPU and you get an additional damper on excitement and corresponding sales.