Because it doesn't matter. They have 2 Mario RPG series already, which already defies all logic, so trying to make a sequel to the original one at this point isn't a particularly great idea. It would take more effort to get Square to work on it anyway. Might've happened had Nintendo not thought of Mario and Luigi before Square Enix was pro-Nintendo again around 2003.
That being said, AlphaDream should be allowed to actually make a new video game, so I would prefer Super Mario RPG 2 over a 5th Mario and Luigi.