Forum OpenACS Q&A: Response to Windows, AOLServer, and OACS
I'm guessing it's a dearth of ISAPI skills. Technically it's certainly not a bad idea, but I think a cross-platform effort like Apache2/mod_aolserver offers more ROI. I also think that your typical Win32 IIS administrator is going to want to avoid ISAPI filters at all cost on their production servers. You're destabilizing a somewhat fragile piece of software (my experience coding for a web farm), and adding new potential security holes.
I also wanted to mention one thing Michael left off his recap list : in the event of going with a native Win32 database, you'd need to maintain the ODBC driver. I'm not sure how big an effort would be required since it's a pretty static spec, but it's something to keep in mind.