its because the very delicate gills collapse and dry out quickly in air when supported and hydrated by water, and thus the fish effectively suffocates. Its the other side of the coin where we can't breathe underwater because the structures in our lungs are not designed to be able to efficiently take oxygen out of the water that is now surrounding them.

However this depends on the fish in question though. some fish are more hardy and can withstand longer periods outside the water. and some fish have special features that let them partially breath air, such as modifying parts of their digestive tract to function as a lung, using their swim bladder as a modified lung, or developing an entirely separate lung-like organ for use. These fish usually live in areas where water quality can become poor. Surprisingly a large number of popular fish kept in home aquariums have these features, such as Bettas, Gouramis, Plecos, Cory Catfish, Oto Catfish, and Ropefish