There’s no one single reason, but the top theories:

1. Tuna oil was a thing before “tuna fish”. Yes, people could have said “tuna” but they didn’t. That’s language for you. People say “ATM machine” and “PIN number”, too.
2. “Tuna fish” has a slightly sing-song pattern to the stressed/unstressed syllables that probably contributed
3. For whatever reason, “tuna fish” tends to refer to canned tuna, whereas “tuna” can include fresh (or frozen) tuna.

It’s… just how language evolves.

I think, however, that “tuna fish” is slowly dying out in favour of just “tuna”. As a 50 year old, anecdotally I have seen the usage decrease in my lifetime.