It's probably hard because all the tiles are shared between games, but slopes weren't in SMB1. I don't think it Super Mario Maker uses the original collision detection, but I know that if they were adding them, then they would have to test every steepness level to so if it works in each game.
Also from experience with SMW hacking, there's a lot more tiles necessary for slopes than solid ground since they have to be able to connect to all the other types of slopes as well as walls and flat ground tiles. Another problem is tilesets, because across every game there aren't slopes in say, the castle tilesets. In it's current state, SMM has the same set of tiles and sprites for every game and tileset. In order to add slopes, they would need both additional programing and a lot of new graphics.
