I notice another politician has come out with another book. One of the premises is that politics isn't interesting to mainstream Australia. If that is so how, do so many politicians get books published and how do so many other mildly interesting non-fiction ideas get up? Looking at some of the agents' websites and seeing the non-fiction titles they're boasting I can't believe enough people would read them. Is the market for fiction tiny and the market for non-fiction endless?
Politics aren't interesting to mainstream Australia but politicians are, because they've made themselves into celebrities. You may wish to read the book Things Bogans Like - or the website of the same name - to fully understand this rationale.
As for the other 'mildly interesting non-fiction ideas' - well, they're mildly interesting to you, but how do you not they're not very interesting to others? Non-fiction books can appeal to almost anyone, whereas fiction, sadly, cannot. Fiction reading is a habit, usually acquired in childhood; it requires patience and dedication to, first, become acquainted with a story and, second, stick with it, especially when there's a lot competing for your cultural attention. Non-fiction reading is often performed for the gleaning of information, and one doesn't need to have developed a habit for it - one just needs to want to know the information. Thus publishers are more prepared to take a risk that their non-fiction books will hit enough information-gleaning targets for the book to make its money back and, every once in a while, make a profit.
So to answer your last question more concisely: the fiction market is relatively tiny compared to the non-fiction market. And just because you wouldn't buy any of the non-fiction books you can't believe agents are displaying on their websites, it doesn't mean other people wouldn't. Books are for everyone, not just you.