This is because all programs that backup the games use homebrew, which cannot be used on official firmware above 1.5. And even if they could, any official firmware above 1.5 can't run backup games anyway.
PSP, all the way from 1.00 to 5.50 GEN D2 - DS Lite w/ M3DS Real (M3 Sakura v1.34)
PS3 3.01 - iPod Touch 2G 16GB Jailbroken - Xbox 360 60GB - PC, Q6600, 3GB DDR2, GTX260 (216) 896MB
PSP, all the way from 1.00 to 5.50 GEN D2 - DS Lite w/ M3DS Real (M3 Sakura v1.34)
PS3 3.01 - iPod Touch 2G 16GB Jailbroken - Xbox 360 60GB - PC, Q6600, 3GB DDR2, GTX260 (216) 896MB
but as far as the complete authority behind them, dunno....
you'd have to have it w/ a legit sony signature attached to it to run it on a legit firmware, which would be much more trouble then downgrading and using ISO.