The reason that people are OK with Mario not having online is like, literally 20+ years of not having Mario online. The games have been pretty good so far, and many people are quite happy to play Mario games offline.
Technology advances, games should advance with technology. Saying "Well Mario didn't have online 20+ years ago!" is not an excuse.
Also only recent Mario entries have been co-op focused, and this is the first real co-op 3D Mario game. So yeah, it's a completely new ball game.