I think All-Stars offers more gameplay opportunities than what Mario Kart 8 does. Certainly Mario Kart 8 is much better designed and polished, undeniable, but All-Stars offered more gameplay modes and variety I thought and at the time, I was hoping that Mario Kart 8 would take some cues from it, instead of just being a standard Mario Kart game, which it turned out be. That's not a knock against the game, it's a perfectly fine game, but I think the Mario Kart formula is slightly stale to a certain degree at this point and its only real big "feature" was anti-grav wheels, which looks cool, but doesn't particularly affect the racing too much.
All-Stars had unique characters with their own vehicles and specials, there was progression oriented mode for all the unlocks (Which got way too damn hard towards the end lol), and alternate game modes. It had multiple vehicle play styles all in one map and some really exciting levels that changed as you played. The vehicle options are what make me want a Diddy Kong Racing 2 so much. If they could do something similar with that, it would be an incredibly epic kart racer introducing the adventure map overworld and bringing back even cooler boss fights.