It was a historical accident, in that 24fps was the cheapest frame rate that looked decent on the equipment at the time. Film stock cost money, and cameras and projectors that could reliably handle higher frame rates were more expensive. If the motion picture industry was starting from scratch today but with all of our modern equipment then I'm certain the standard frame rate would be higher.
Not really. It's not just some historical accident why we ended with 24fps (or close) and not 50fps or 5fps or such.