Why did the 1953 Corvette come only with a powerglide?

The 1953 Corvette came only with the Powerglide automatic transmission because Chevrolet, at that point, didn't have a suitable manual transmission that could handle the Corvette's then-new, relatively powerful (for the time) small-block V8 engine. They needed a transmission that could withstand the engine's torque, and the Powerglide, while an automatic, was the strongest option available to them at the time for production. A manual transmission capable of handling the engine's power wasn't developed for the Corvette until later model years.