Voices change over time there's no way Ozzy or most singers can sing a song in the same key they did almost 40 years ago. It's just not humanly possible. The male voice goes through 3 or 4 significant changes during a persons life. You can't change that, it's just a natural thing.
Bands need to change keys for songs over time so it's easier for a singer to do them and they don't blow out their voice. Can you imagine if Rush still did Temples Of Syrinx in the same key live as on the original recording? Geddy would've lost his voice years ago.
It's the same with Ozzy. The key needs to be changed so he's still able to perform every night.