Money. A lot of silent movies were filmed around 16-18 to save on film costs. Also sometimes they'd crank it up or down for dramatic or comedic effect so a film could have a variable frame rate.
Then along came sound with the audio track printed on the side of the film and they had to bump up the basic frame rate to 24 (or 23.9something) which was the lowest they could go without it sounding bad.
Money. A lot of silent movies were filmed around 16-18 to save on film costs. Also sometimes they'd crank it up or down for dramatic or comedic effect so a film could have a variable frame rate.
Then along came sound with the audio track printed on the side of the film and they had to bump up the basic frame rate to 24 (or 23.9something) which was the lowest they could go without it sounding bad.