Understandable from a developer's point of view. You need to focus your ressources to some platform, and as MSFS already sees more activity than P3D and X-Plane combined (50-75% of flights, depending on which statistics you look at), it's quite logical to develop for that sim if you want to maximize your chances of making a profit.
Of course it's bad for all of us who don't use MSFS as their main sim (I haven't even tried it), but as a dev - at least one that needs to make a living out of addons - you need to prioritize.