Universal mastering frame rate.
24p where “p” stands for “progressive” is a universal mastering frame rate which allows easy conversion to other frame rates used around the world. Motion picture film has used the 24p approach for years. Video suffers from multitude of standards used in different countries (sometimes in a single country) so creating a master that will convert well into other standards is of great importance.
It is always possible to make standards conversions but they may come with a quality loss. The frame rate is not the only property that determines how a master will convert but it is arguably the most important one. Masters created at 24p (or more often 23.98p) are appropriate for versioning to any existing standard with the least amount of quality loss.
It is also important to consider the difference between 24p and 23.98p (more precisely 23.976p). True 24p HD is rarely used in the United States.