The appeal is what Mario has always been to me - gameplay mechanics and level design. I'll concede that this may not be interesting to a majority of people, but Nintendo games have always immersed me due to the consistency of their in game universes. Once you get the controls down, you can apply the lessons to any level, and things just work the way you expect them to. It's a level of intuitiveness that I have not seen anywhere else. Yes, that's replays after the first hour, but that's not an order of magnitude different to game length in the 8/16 bit era.
You are right that there are many ways to play Mario on many different devices, and I'm not arguing that focused gameplay in front of a big tv isn't the best way to play for extended hours, but for ten minutes waiting in a queue or on the subway (when there is an internet connection available - don't get me started on that because I'm not a big enough fanboi to be happy with that situation), it's much more convenient than carrying along a 3DS. I think Nintendo did their homework on how people will play this game. It's accessible enough to play in short bursts, which suits mobile gamers.
I do respect that you don't see any depth in this. That's fine. I know that enjoyment of games is very subjective. I'm biased in that I grew up in the golden era of 8 and 16-bit 2D games, so I'm aware that this type of game has more appeal to me than others. (Nintendo probably knows that my demographic is more likely to convert to a purchase as well, but I digress) I've tried to stay engaged with recent stuff on Playstation, but triple A titles require large time commitments and have lots of grind, and hence don't appeal to me. This does.