It's not about laziness or technical issues. It's about Nintendo wanting to control how we play games. They're very stubborn, so if they think there's an "ideal" way to play with your friends (i.e. in a room together) they don't want to give you the option to do otherwise. This stubbornness extends to most of their series. They've made more decisions than I can count based not on what we want but on what they want us to want.
They're getting a little better, but I wouldn't count on online Mario any time soon.
Currently playing:
Bloodbath Paddy Wagon Ultra 9