I spend a lot of time on writer's sites. One site in particular is full of aspiring and already published writers. And I notice a lot of the aspiring ones doing something that drives me up the bloody wall: Claiming that their book has a "built-in audience."
I didn't pay much attention to this phrase until someone said their supposed built-in audience was Roman Catholics. I'm Roman Catholic. I had no interest whatsoever in this person's book. It was about history. I read fantasy, and urban fantasy in particular.
Since then, I notice it everywhere. One person's novel has a built-in audience of knitters. Another has a built-in audience of Chinese Americans. Another has a built-in audience of cat lovers.
Your book has exactly one built-in audience: People who read the kind of things you have written. The reason your book doesn't have the built-in audience you thought it did?
1) Not everyone in that "built-in audience" reads.
2) Not everyone in that "built-in audience" who reads reads fiction.
3) Not everyone in that "built-in audience" who reads fiction reads your genre.
4) Not everyone in that "built-in audience" who reads your genre will like your book.
Saying you have a "built-in audience" is a lie. You're either lying to yourself, or you're lying to the agent you want to represent you. Lying to either is not a good way to make a living.