1 reason is the fact that there are very limited numbers of SLI Opty Workstation boards, or one's with even 1 PCI-E slot. The fact that Opties use ECC Memory (in my experience, disabling ECC on Opties has decreased performance) also means a small loss in performance, and trying to find high-performance RAM in Opties isn't as abundant as Desktops. From the experience I've had with all the Opties I've built and used, their performance on CPU only has been superior to desktops, and using SLI (on the 1 board I have) the performance beats desktops SLI by a small margin. And the fact that most people won't drop $2000+ for a Dual Opty setup w/ a $450 SLI mobo and have to spend extra for ECC RAM (1 friend of mine did, he loves his 2.4GHz Opties, heh).
~~Mad Mod Mike, pimpin' the world 1 rig at a time