I think this is because the muscles (meat) in a fish is very soft, less dense and has high concentration of proteins. The bacteria will easily work on the soft tissues faster than on the hard muscles of beef. The same can be said of internal organs of other animals like cattle where the tongue, lungs, liver, eyes, intestines, and other organs need to be consumed within the first day of slaughtering the animal.
You may not want to read/hear this but please try to find out what is it that they do with tongues, lungs, livers, eyes, intestines, and other organs when people dies - if you are in remote areas you can give them 3 to 5 injection of kerosene in the stomach and bloodstream, otherwise the situation will become unbearable.
How many days can you keep fish in a fridge and expect to get it without that smell that we all hate? May be 2 days?