The hardware vendors used to pour more effort into the Windows drivers than into the other ones. (I think it's not about being unfair or against FSF ideology, it was just that the market so far was still heavily in the Windows' courtyard, so they were kind of forced to give in and have the other platforms only on lower priority.)

I would think a big reason is that the version of Windows that ReactOS is based on hasn't been supported in about a decade or more, and there are people who could be perfectly-well served by it.

Windows' biggest competitor in the past decade or more has always been the previous version of Windows. The early versions of NT were incredibly stable and did a great job while using 1/100th of the memory and disk space of what Windows requires now.