The 12 hour system is about 6 times older than the older than the 24 system. Ancient Egyptians had sundials that divided the daylight exactly into twelve hours, with exact length of an hour changing with the seasons. It wasn't until there were mechanical clocks that it made sense to fix the length of the hour and let the sunrise/sunset times drift.

In some sense, there just hasn't been enough reason to change.

The 24 hour system very much is popular and standard - just not in the US.