Because there's no air to breathe. To breathe in space, Superman would have to take his own air supply off Earth, in which case he'd be as able as anyone else.

Can kryptonians breathe underwater?

Enhanced Underwater Breathing: Is the ability to stay Underwater for long periods of time. Kryptonians have this ability. Aquaman can also stay underwater for long periods of time. Doomsday can also breath underwater for extended periods of time.

Can you breathe on Krypton?

Health effects of krypton

Inhalation: This gas is inert and is classified as a simple asphyxiant. Inhalation in excessive concentrations can result in dizziness, nausea, vomiting, loss of consciousness, and death.