Why not?

This sort of thing only appeals to a small amount of people. 

2D Mario is niche now? O_o

I thought everyone knew that this game was going to be huge. Probably the most selling game on Wii U this year, depending on how well Splatoon does.
Mario Maker is like New Super Mario Bros., but better in every single way.
U have endless levels to play, new power ups, new plattforming elements and so on....
Also: It's a Mario game!
Of course it's gonna sell well.
It will definitely sell several million copies.

No, but making and designing your own levels is. 

That´s because editors are usually boring and unintuitive, and many times the gameplay doesn´t stand up, but we all know Nintendo always try to do everything on the most intuitive and addictive way possible. And is mario, the gameplay is good for sure. Even if you don´t like to make levels, you can enjoy the crazy creations of the people.

2D Mario appeals to a great deal of people; it's familiar and simple to understand.  I think a lot of Super Mario Maker's success comes down to the tools and ease of use when designing and it really excels in those areas. 

But Nintendo needs to advertise the heck out of this title... and advertise it well.  WiiU probably won't get a price drop announcement tomorrow due to a lot of potentially disgruntled Splatoon buyers so soon, but when SMM launches this fall they need to do a bundle/price drop to $249.  I think this title could do much better for WiiU than many people expect.

I've been waiting for Nintendo to make a Mario level editor for many years. I'm happy it's finally coming.

I know 2D Mario is very popular, I just don't think people will be that interested in making their own levels. But maye I'm wrong. 

Same here.

I know I won't touch the map editor, but I'll be playing the crap out of challenge maps.