Lots of reasons. I am seeing folk that are just learning about this so keep asking why!
I'm seeing a Xeon W3690 for 1,079 dollars v a Xeon X5680 for 1,639 dollars both at Newegg.
The W3690 has a CPU passmark of 10,249 while the X5680 has a CPU passmark of 9,838.
Clearl the 3690 is superior to the 5680 so why the odd price difference?