An MA map is very different than any of the other maps used. They are not just swap and play. From what I understand, a map used in the FSO would have to be totally redone from scratch. Also, those going into an FSO know what they are heading into, the MA are for everyone else, including those newbis trying to get off the ground the first time. They can't have all the "extras" in the mains because it would make it to tuff for the new guys, turning them off and possibly loosing a new subscriber.
I'm only guessing, but the reason they might not allow all these different terrain tiles ( desert, snow, tropics) might be due to file size. They would have to have all the new tiles added to the main file so newbis wouldn't have to download the game, then a terrain file, and install that. Also, some of the terrain file that you see in the FSO type maps have a lot more polygons and so take more computer to run them, again, they have to keep the game as clean as possible to make it playable for as many as they can. It is a business after all.