Not running its extensions was mostly a result of the fact that that required running IIS on Windows NT. IIS had a number of high profile problems, and Windows NT 3.51/4.0 (remember, bare metal not VMs) really wasn't ready for running at that scale or in that context. Compared to putting in a SPARC-based machine running Solaris, it just didn't make sense - much harder to manage, less efficient use of resources etc.