Linux and Mac support is better that single player?
Yes. SpaceEngine is, first and foremost, a free educational planetarium program. Games will come only AFTER that part of the project is completed (which will also mark, approximately, the point at which the engine will be sufficiently complete for full game development to begin). SE needs to be usable by as many people as possible, so it makes sense to prioritize cross-platform. It would also be beneficial for the commercial game products to be released on multiple platforms simultaneously, rather than just Windows.
Considering the above, and considering that porting the engine to Linux and Mac would take MUCH less time and effort than developing a full game, it makes sense to do that first.
the donations rate goes slower...I guess that is because only Linux and Mac Fans use the donation button, mostly.
No, it has decreased because there has been a long time since the last release. After the next release the donation rate will probably increase again.
Also, there are several threads where this could have been discussed. It was not necessary to create a new one.