Reason being is because well, though all three games were thrown on the same disc, that doesn't mean things could have made the games better. In DMC 3, they could have at least thrown in Vergil's side of the story (wihtout going through the same missions as Dante), added the combo's left out in the PS2 version for Vergil and probably alot more. Even though I could be wrong on this, I think it would have been better if they would have just done the HD better or just made it like Halo Combat Evolved Ann. which was amazing because they did a truth graphic overhaul on that game 
