What you're seeing is what that agent specializes in. Typically, they'll specialize in the type of books they like to read (though not always... that said, my experience with an agent that didn't specialize in what I wrote didn't go so well).
As an example, I cannot read romance novels. Can't stand 'em. If I were an agent, I would state "no romance novels". It has everything to do with what they're passionate about and can recognize as good or bad, plus the editors and connections they have.

Good luck!