Question

why does the fish die when kept out side the water

Answer 1

because they do not able to breathe outside water

Answer 2

Not all fish die when taken out of water. Unfortunately, we are tutored to believe that fish only breath through their gills, which is not true at all. Predominantly, fish use the following organs to breath.

1) Gills - Gush water through the gills and absorb the oxygen in it and leave the water.2)Skin - Absorb oxygen present in the water through skin
3)Buccal Cavity - Simply meaning mouth. Some fish do breath through their mouth.
4)Lungs - Certain fish have fully developed pairs of lungs.
And they do breath out of water also. These fish typically come to the surface of the water once in every 20 minutes and breath in open air and go back to water again.

Certain fish can use a combination of the above organs to breath.

Now, Fish that use Gills and Skin die when taken out of water. However fish that use Buccal Cavity and Lungs to breath can survive out of water. And Some do survive even to the extent of 3 to 4 days out of water. The best examples of this are Murrel (Ophiocephalus Genus), The Giant Cat Fish (Magur in Hindi, Clarias Butrachas Species) The Mud Crab (Sylla Serrata)