24hz is the lower limit. People will perceive 24hz as a smooth sequence, especially with motion blur, while anything below it will start to look choppy. Of course humans can perceive higher frequencies. But 24hz became the standard because celluloid film is expensive especially in the early days of cinema. The less frames you need to shoot the less film you need to buy and develop. And film back then was probably not sensitive enough for the lower exposure times that come with higher frame rates.