Probably inertia. Windows 3.0 and 3.1 didn't ship with a TCP/IP stack. You had to either have something like Trumpet or install Internet Explorer 2.1 (which, AFAIK, was a separate purchase) to get Winsock. Or you could try to use DOS drivers. Even Windows for Workgroups 3.1 only shipped with NetBEUI and IPX/SPX. It wasn't until Windows for Workgroups 3.11 that the OS shipped with TCP/IP.