Here's the sneak peak
To discuss why, we'll have to flip through some history.
From Merced to Tukwila
Intel began work on what would become Itanium back in 1994 in a joint venture with HP. The two companies chose to pursue a design philosophy they termed EPIC (Explicitly Parallel Instruction Computing). As an EPIC processor, Itanium pursued a very different design philosophy compared to the Pentium Pro and the other out-of-order execution processors that followed it.
Instead of using specific CPU hardware to re-arrange and optimally schedule instructions for execution (defined as Out of Order Execution, or OoOE), Itanium relies on the compiler to optimize code at run-time. This allowed the designers of Merced (the first generation Itanium) to devote more die space to execution hardware, thus boosting theoretical performance. The weak link in the chain was the compiler itself. If it failed to detect and exploit thread-level parallelism at runtime only a fraction of the CPU's execution units were in use at any given time.
Merced, the First Generation Itanium Processor
This was logical given the chicken-egg dilemma of introducing a brand-new architecture, but it was negative ammunition all the same. Most damaging of all was the way Itanium had been initially marketed. When Intel announced it was targeting a 1999 launch window, respected analysts were soon predicting that the chip's combination of 32-bit compatibility and advanced 64-bit execution would sweep the length and breadth of the x86 industry. The chip's initial weaknesses were significant, but its marketing was worse.
We're not kidding when we say the chip was poorly misrepresented/marketed. The predictions above were made by IDC. Original image courtesy of Wikipedia
Over the last ten years, Intel has refreshed and updated the Itanium core multiple times. The last significant refresh, Tukwila, was built on a 65nm process with up to four cores and 24MB of L3 cache. With Poulson, Intel is leapfrogging 45nm entirely and moving Itanium to its cutting-edge, 32nm process.