I find it odd that Christian music is categorized by lyrics, not music

As a fan of black metal..

Burzum and Gorgoroth are two different bands but fall under the same category.

Christian music is about the message. Its basically all talking about the same thing.

I understand that. The point is that is opposite of how all other music is categorized.

If I hate country music, I’m not going to all the sudden like it because of the lyrics.

Indie music is characterized by what type of record contract the band has. That's even odder to me.

There are only 2 genres of music. Christian, and SATANIC!

Which keys, notes, chords, and rhythms are "Christian" and which aren't? Instrumental music has no ideology, it's universal. I can't conceive not getting that.

It's consumer driven. Move like movie or book classification in this case.

If you like a certain style of movie cinematography, style of acting, a book that is more about character development, or a book that reads more like its the work of a storyteller, you probably have to dig a little deeper than the "drama" category for a movie or "it's a dystopian story" for a book.

So people look for christian artist then delve into the music styles.

The radio stations almost have to play all these types of music because, while the industry as a whole is pretty healthy, no real subcategories would generate enough interest to financially sustain a station in the places they tend to exist. so they're basically Christian pop stations with a smattering of soulful gospel stations and the occasional bluegrass/traditional gospel stations.