Windows is a bad product made by a company that has historically been a bad and unethical actor that has attempted to limit user freedom and destroy freedom of choice by illegally destroying competitors. Recently we are to believe that they have found jesus and ethics via quiet contemplation and peaceful regime change.

All projects are ultimately created to scratch somebodies itch. If it doesn't work on windows out of the box that isn't their use case. What you are wondering is in effect is why people don't pay money to purchase a windows license which will ultimately fund a bad and evil company in order to enable the projects software to run on an inferior OS that the dev doesn't run or care about. If its not end user software for desktop users it doesn't even have the positive effect of enabling a substantially bigger group of potential users to benefit from the software. For anything server related they are either already running linux or can as easily run a linux vm as a windows one.

Further the users who would benefit will by and large buy a license if the software is non free but probably wont contribute anything but complaints phrased similarly to the support requests they would make for paid products that had failed to perform adequately.