The original disc contains some codes that the ps2 dvd-rom reads and if these codes are on the disc then the drive understand that its an original game and the game starts,
These codes cannot be written on a cd-r media and therefor a backup game does not start, because it does not contains the codecs
To fully understand that i will explain you how game shark works.
Game shark is a pressed dvd-rom that is used to boot any game.
So when you want to play a backuped game, you first boot with gameshark in order the ps2 dvdrom to read these codecs and then you prompt to insert the backuped game