Quote Originally Posted by Verno View Post
Was it ever stated in G1 exactly why his spark is inextinguishable?
Because he still had a toy on the shelves and Galvatron had inconveniently killed him.

Sparks weren't even invented until Beast Wars, so he didn't have an "indestructible spark", he just - understandably - happens to be a ghost in a story where Octane found a Decepticon Crypt and then the ghost plot point was used again in Ghost In The Machine.