Not everyone had the opportunity to play these mario games in the past, so for many It will be a new Mario experience for them. Some never owned the original hardware the games run on and some want to revisit these games for purely nostalgic reasons. I, for example haven't played Sunshine in over a decade, and it holds many nostalgic memories for me. That's why Im interested in it.
Emulator's aren't everyone's cup of tea. Some find it a headache to emulate games on their computers, this serves even more true after the generation of the N64 and PS1. Not everyone owns computers that can run emulate or run games smoothly. I for example use an old HP. All I did was replace the HDD with an SSD. It was never meant for gaming anyway, and I can't afford to splurge money on anything better. Not to mention, some gamers encounter issues with emulating games and give up, not everyone is good with computers. Anyways, I think lots of people enjoy the feel of playing their games an actual console instead of emulating it.
I admit that I don't feel excitement when it comes towards SM64. The modding and emulation of that game has been done to death, therefore potentially spoiling the fun and expectations one would normally have for Nintendo's port. Just an observation I'm not saying emulation and modding is a bad thing.