The relative proportion of people who purchase Super Mario Maker and actually use it to make worthwhile content is probably going to be pretty tiny. But even that's probably going to be huge compared to the number of people who would have the knowledge and capability of getting SMW running in an emulator, much less find and use Lunar Magic. (Is that still what everyone's into these days? I don't even know.)
IMO, I think they saw an idea that would be relatively cheap to produce (I believe all the materials used in SMM are from previous games, so no new stuff persay)
I wouldn't think it was particularly cheap to produce. Making a UI that people actually want to use can be tremendously difficult. Anyway, things like spiny helmets and cannons (and lakitus) that can fire arbitrary objects are kind of new. It does occur to me that Super Mario Bros. X had a little something similar . (I'd still like to take a swing at Super Mario Bros. X eventually.)
...On that note, it does
seem reasonable to suggest that Nintendo might be trying to head-off someone making a PC-based game creation utility that might capture the popular imagination and drag people away from the Wii U. I understand they did shut down Super Mario Bros. X, and they're probably aware of Mushroom Kingdom Fusion (which seems to be doing a good job of shutting itself down).