I think it comes down to the target audience. Of course both Mac and Windows have their fair share of users across all categories. However, it seems that, in the US, most devs and enthusiasts prefer to use Mac/Linux, while Windows is used a lot by gamers/office workers/etc. (not alleging that enthusiasts and devs don't use Windows, far from it, just talking comparatively). Given the price of an average Mac being higher than a Windows machine, it would also be fair to suggest that Mac users, on average, tend to have more disposable income as well.
With this in mind, it makes sense that the platform with more devs with more disposable income would be a better target to sell niche indie software as well.
For a similar phenomenon, look up the breakdown of earnings from apps on iOS vs. Android. There are plenty of analysis on this over the years, so here is a recent article I just found as an example[0]. Not even diving into the actual revenue per user or per app, iOS apps generated overall almost twice the revenue of Android apps in Q3 of 2019 (using numbers from the article I found), despite there being more of both Android users in the world and Android apps on the store. My reasoning as to "why" boils down to a simple guess that whichever device has more users with higher disposable income is the one that is gonna bring the developer more revenue.
0.https://www.techaheadcorp.com/blog/android-vs-ios/#:~:text=A....