Why do penguins lay one egg per year?

this depends on the species of penguin. A penguin only ever lays one egg at a time, but some species lay two eggs several days apart.

The Emperor Penguin and the King Penguin lay just one egg. All other species lay two eggs, although in several species, the first egg often does not hatch or is not incubated. This is the case for the Erect-Crested Penguin, the Macaroni Penguin, the Royal Penguin, the Snares Penguin and the Rockhopper Penguin. Galapagos Penguins may lay either 1 or 2 eggs.