Why don't writers write in the genre they like best?

My bf is an author. He self publishes books for the Kindle. They're all 12 chapters long and pretty much follow the same plot: Murder is discovered. There's a bunch of suspects. The most likely suspect is zeroed in right away, but is always innocent. The real murder is smug that he won't get caught until the detective reveals the murder and the undeniable proof, and the murderer is arrested.

The thing is, when I borrowed his Kindle a little while back he has almost no mysteries on it at all. He had Defending Jacob, which he said he hated. He had And then There Were None, which he says is probably the best mystery ever written and then he has another Agatha Christie The Orient Express which he said was highly over rated.

He two books on how to write a mystery, and a lot more on how to write fiction as well.

He has a lot of legal thrillers, which he told me are not mysteries. He has more books by John Grisham than any other author.

He has a lot of westerns. When we were moving his mother into a new home, she had boxed up the stuff he had at home and almost all the paperbacks he had were westerns: Louis L'Amour, Zane Grey, and Tony Hillerman.
I asked why he doesn't write westerns and he said nobody buys westerns anymore.
Then he said a lot of writers don't read in their own genre. He said a lot of romance writers hate the genre and despise their readership.
I asked why not and he said Don't know. I guess you get burned out faster.

Are there other reasons?

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