This answer is a supplement to durron597's answer - which is completely correct and should be the accepted one. I'm writing this answer because there are several misconceptions floating around about the meaning and the implications of Campbell's monomyth, and I want to clarify them. This answer also gets at the original question -- why is the monomyth so common.

(I'm not going to cite sources in this answer because this is my own interpretation of Campbell, and the arguments I'm using are probably different from Campbell's own. However, I think my interpretation is useful and will help people understand Campbell.)

The monomyth is more interesting to people who study literature than it is to people who study religion. This is because it is fundamentally about how stories are composed and not directly about religion, though Campbell (successfully) argues that stories are a key component of religion.

Although most people describe the monomyth as having 12 steps, I think of it as a two step process:

The hero is put into an unfamiliar situation or faces an obstacle. The hero learns to overcome that situation/obstacle (or just learns something) The hero could be compared to a child/adolescent learning to navigate the world.

The reason why the monomyth is so common is that the two step formula I just described is one of the best ways to tell a story; some people would say it's the only way. If you are an author and you want to teach your readers a lesson (e.g. how to live their life), a great way to do that is to create a story where the hero faces a challenge (e.g. my life is not meaningful) and then overcomes it (e.g. the hero finds meaning in their life). Because people place themselves in the situation and see through the eyes of the characters they read about, the reader will then learn from the fictional characters and learn how to overcome similar obstacles in their own life.

As Campbell notes, the monomyth is frequently used in religion, but I think that has more to do with the fact that religions use stories than with religions using the monomyth.