probably because the 2D kept selling well, and the 3D Legends bombed. So they skipped the 3D, and kept at 2D, until X7, where again they tried 3D, by integrating it with 2D. It failed. So back to 2D it was.
Command Mission was 3D, although not a platformer, but it probably didnt sell well or they didnt want to take the risk of trying 3D again, if there isnt a CM 2 or another 3D effort.
So they went 2D with X8. buuut, X8, and ZX2, didnt sell well enough, so the 2D began to stagnate. With no idea left on what to do, they figured a return to Classic's roots would be a welcome revival of the classic franchise.
then they made another one.
but apparently 10 didnt do well enough since we dont have 11.
And so we are where we are now, where Capcom doesnt know what the [tornado fang] to do with megaman and doesnt want to take the risk and try something. So they neatly fold him away and focus on merchandise instead. Until someone gets a lightbulb over their head and releases another megaman game, one which will be received as "FINALLY A NEW GAME" but which will probably be nothing all too special at all.