Because Sonic Adventure, and subsequently Sonic Adventure 2, are some of the most ambitious games ever made, maybe a bit too ambitious for its own good. Across these stories with gigantic scopes, we're treated to 6 characters each with their own play style and piece of the story that ties into their own personal character arc. Even down to the music, each character has their own musical genre for whatever stage they appear in.
In SA2, we go from cityscapes, to jungles, to deserts, and then to space in a globe-spanning epic about a revenge plan 50 years in the making. There's secret agents, treasure hunting, high speed chases, and infiltration mission into the worst weapon in human history. There's laughter, tears, sorrow, and above all else, a feeling of hope that permeates through the dark, because there's no way he'd let anything destroy the world that's as cool and as blue as he is! Because the gameplay is so varied, you're bound to find something that you like, and if not, you won't have to do it for too long before something new happens.
And if you want to take a break from it all, just relax and raise some chao! Teach them karate, or racing, or even how to draw. Heck, YOU can race with your friends in the mutliplayer options and kart race mode! There's something for everyone .
Also probably nostalgia. Honestly, I think people liked the Adventure series for what it represents. I think the Adventure series is much better in theory than it was in execution, but there's still something there. I think a potential remake could give it the polish it really needed.