1. Microsoft got paid about $400 million for their work on OS/2, supposedly, through to v1.3 .
Development of Windows through to v3.0 was much much less. It is safe to say that Microsoft made money from the partnership with IBM.
2. OS/2 installation had a fatal flaw: if your PC had cache memory you had to disable it while the installer copied the files from floppy. It didn't hurt install speed since floppies were the bottleneck but it was very confusing to the newbies to OS/2 that IBM was trying to attract.
3. The Ziff-Davis magazines were pay-to-play in editorial content and they plugged Microsoft as being the better choice all the time. And pre-Internet they were a big source of information.