As a steady diet it is bad because it lacks the essential amino
acid taurine, without which cats may go blind. There have been
cases of people feeding their cats tuna thinking it had to be
"better" than cat food, only to wind up with blind cats.

Other reasons, just guesses: high mercury levels, which are also
what make too much tuna bad for humans; and the fact that the
sodium level is so high. I used to use it to lure my cats out of
hiding or in to the house when they escaped, but it makes
both of them throw up now. The vet said it is "too rich"
for them, whatever that means.

A cat has to use a vitamin (A, I think) to digest tuna. So, if the cat
is fed tuna lot, it will end up deficient in the vitamin, which is a
serious problem. Cat foods that use fish products will add extra of
the vitamin to solve the problem. So, cat food with fish is fine. But
plain fish is not, at least as a staple. Occassional treats are fine.

It's not that tuna is bad for cats - as an occasional treat, but it
doesn't have all of the nutrients they need for a basic diet.  IOW -
popcorn isn't bad for humans - eaten in moderation. As a snack, it's
decent. But if it's *all* you ate...

Cathy