See another recent thread for my theory (based on what I heard from various Unix vendors at the time) that this was mostly due to fear of MS patent litigation.

See another recent thread for my theory (based on what I heard from various Unix vendors at the time) that this was mostly due to fear of MS patent litigation. iirc AIX did have an IOCP-like feature. MS acquired the rights to DEC's patents in this space, handy since IOCP is similar to VMS's QIO.