If I remember well, it's because of the "original" discs being pressed and the copy being burned. There are some sectors on the original that can't be read (not too sure about that) and the same sectors can't be written to with a burner on the copy.
If I am right (and if I'm not, I must be close to it ^^ ) then those sectors of data probably are authentification data. Without those little bits, the Ps3 sees a Data disc, but not a Ps3 disc.
There are probably other reasons but I am no expert.