Primarily because they've evolved breathing structures called gills instead of lungs. Lungs respire air. Gills must pass water across membranes to extract oxygen and off-gas carbon dioxide. Gills don't work outside water so they can't breathe. Their skin is not keratinized either so they get dehydrated very quickly.

One exception is the lung fish which has lungs for respiration and lives in swampy mangroves.

Manomay Shravage


When we take the fish out of the water its gills dry up and this prevents the entering of water into it. Thus it suffocates and dies.