Right, standardizing on 25 fps in Europe came from PAL's 25/50Hz rate.
I think 24 fps for sound film originally arose from the requirements of the optical soundtrack: slower running 35mm film wouldn't have enough resolution for decent sound quality. That's my guess anyway.
Silent films were often shot on slower rates around 16-18 fps (which explains the widespread comic "speed-up" look for video copies of silent movies, as the transfer was done the easy way by playing it back at 24).
Actually it has to do with how film is shot. Lightbulbs pulse at mains frequency, shoot indoors at 24fps in 50hz land and you'll get a horrible 1hz strobe effect.
There are tricks you can do with shutter angle that negate this, but they are beyond the scope of this comment.
I think 24 fps for sound film originally arose from the requirements of the optical soundtrack: slower running 35mm film wouldn't have enough resolution for decent sound quality. That's my guess anyway.
Silent films were often shot on slower rates around 16-18 fps (which explains the widespread comic "speed-up" look for video copies of silent movies, as the transfer was done the easy way by playing it back at 24).