Because Sonic Adventure 2 was great? It wasn't. How many people liked tails' or Knuckles levels? Who liked the treasure hunts and the boring repetitive shooting with the annoying "BEEEEEEP!!!" whilst holding the button? I'm guessing not many people. People only liked sonic/shadow levels, and people loved the Chao. Then they replayed their favourite levels to get canisters and animals for said Chao. Then people loved it more because they had played the levels so many hundreds of times getting everything that they would cream their friends in multiplayer, normally by automatically jumping to metal sonic. Don't worry, I'm guilty of nostalgia goggles myself. Sonic adventure wasn't a good series. Sonic Adventure 1 was even pretty naff, big the cat was a joke.