This is probably because he was the only character to get his own video game on the PS2, Xbox and Gamecube. Another reason could be because he can easily fit as Sonic's rival, since he has super speed like Sonic. Knuckles was a good rival too, but he was too friendly and he couldn't keep up with Sonic's speed. Shadow also became the most violent and mysterious Sonic character. You know, SEGA was going to make Sonic Adventure 2 Shadow's only and last appearance, but when he had a huge amount of fans, they decided to make appear again in Sonic Heroes.