Main reason is that sharks without dorsal fins can't swim, and so sink to the bottom of the sea straight away. If it sank how did the fisherman catch it? According to the article it was caught on a baited hook attached to a buoy, can you honestly see a shark in that state, that cannot swim deciding to have a wee snack?
There are other reasons too - the shape of the bite is different, the fact that the shark was still alive and not eaten by whatever "monster" did that to it.
Maybe it happened after it had been for a 'wee snack' and whilst it was caught on the baited buoy is when it was 'attacked'?
The shapes of the bites aren't that different, the smaller bite on the dorsal fin could have been a more tentative bite and the main one was on the underside.