Originally Posted by SATCHMO

If you look at things from the perspective of how limiting a gwnre's criteria is, then you get a more accurate answer to the question.

The most common criteria for metal is heavily distorted guitars, minor key signatures and a dynamically heavy beat and usually brooding or somewhat angry vocals. Obviously there are some exceptions to this. That puts a lot of restrictions on the amount of diversity within the genre.

Contrast that with pop music, which essentially is almost solely defined by it's formulaic song structure, which is ubiquitous in the world of modern music, and somewhat by it's lyrical content, which is also ubiquitous in modern music, and you see that the genre is not nearly as exclusive. You can have a pop song in a modern key or a minor key. You can have a pop song with distorted guitars, or clean guitars, or no guitars at all.

I think the confusion comes from the fact that the word "pop" has become a catchall term to mean "mainstream", and while most mainstream music does fit within pop's formulaic song structure, not all pop is mainstream, not by a long shot.

Metal on the other hand given the amount of criterial restrictions does have an insane amount of variety, but it cant compare to pop, which is virtually a blank canvas as far as composition goes.