24 fps doesn't go evenly into the 60hz refresh rate monitors most people have.
As a consequence, your video player has to do 3:2 pulldown which blurs frames together and screws up the clarity of video.
This is the reason why 24 fps video looks better in movie theaters than at home - the projectors can display the video at native 24 frames per second.
120 or 144 hz monitors divide evenly by 24, so each frame of the source video is displayed for a whole number of frames instead of the two and a half frames it's displayed on a 60hz monitor.
Some televisions and monitors also support 24hz (film) mode which solves this issue from another angle, as well.