That's not to downplay Blender, but Autodesk pay full-time developers to do that and freeware developers can't devote the time and resources a paid programmer can.
Blender ist not just developed by a bunch of nerds in basements. In fact, there is an entire foundation behind it to fund core development by means of hired, paid professionals.
en.wikipedia.org
Blender's huge advantage is that anybody can contribute to its source code, patch and compile it at any time for any platform. Improvements to 3ds Max are limited to the yearly release cycle and the odd service pack and is completely constrianed to Windows, excluding a lot of the creative industry that is using MacOS- or even Linux-based systems.
This might not play a role in an exclusive MSFS' ecosystem, but any addon developer considering targeting multiple platforms like MSFS, P3D and X-Plane is better off with Blender as the source file conversion step with the associated loss of information (mesh smoothing, UV maps, animations) can be avoided.
Although things may look very different with MSFS 2020 development in a couple of years, Max is going to be a preferred tool for "big" companies for now because Max is explicitly supported by the SDK. We freeware developers very much hope a freeware or low-cost modelling option will continue to be supported long-term.
As long as FS20's file format specification is publically available, nothing can stop anybody from creating exporters for any modeling tool out there. This was the case for FSX and that's the reason why there is a Blender plugin for it.