Because Nintendo didn't do that, some goody two shoes did trying to protect them. Or maybe Softendo did because it was a "better" version of Mario Forever. In my opinion it is far better.
Update: Upon research I discovered the "goody two shoes" is an anti-piracy company in the United States but what that doesn't solve is why did they want files that mostly unusable to anything but SMBX taken down. I say mostly because there are a lot of custom graphics but many fall under fair use as they are custom made therefore technically artwork and NOT Nintedo's IP, to fit SMBX's graphic requirements for some graphics any actual rips would have to be altered to fit. The rest of the files are npc files, level files, and a world file that can only be used by SMBX/SMBX2.