Mac users are willing to spend more for software per average (especially for high quality), so the best funding source is systematically excluded.
More per person, yes, but there are also not many Mac users in total. Even if a Mac user is willing to spend $20 for something a Windows user will only spend $7 for, the fact that there are 10x more Windows users than Mac users means that focusing on Windows would still be more profitable.
Anyway, that is not what determines the pace of development, so it's irrelevant.