That's because when the game is 3d with movie like graphic qualities, it becomes almost like a movie and can be incorporated into a game movie trailer.
I think some old mir trailers also had snippets of actual game play incorporated in them, so you can always argue. Point is, I have never played a game or watched a movie because its trailer presentation made it look tempting to try.
Mind you, 3d game is not really a mir game anymore. Mir game is defined by isometric game view and for example, mir3d Eternal barely qualifies (not because you can tilt game view to FPView but because mobs are 3d models which has consequences on gameplay). After working on mir3d Eternal game for some time, whenever I return to mir2, it is like coming home from foreign lands.
Fully 3d games cater to a whole different game players, never mind that some pple play both 2d and 3d. Very basic difference is that in mir game the map orientation is fixed while in all 3d games the map rotates.