At least once a day, I have to reject a book purely based on genre.
Our agents all keep me updated with their preferences. If you send a memoir to Agent X but she’s not repping them, I either send it to Agent Y who does rep memoirs, or reject it because none of our agents do.
Agents’ preferences are a matter of (a) what is selling at the moment, (b) what doesn’t conflict with their list — if they have three PNR’s, they might want to diversify — and (c) what they’re interested in. These preferences aren’t always reflected on agencies’ websites.
You might receive a rejection because your book came at the wrong time. Elemental magic, for example, is a no-go thanks to the recent end of Legend of Korra and the tribute novels that followed.
Or maybe the agent doesn’t rep a certain genre anymore–even though the website says they do.
Don’t take rejections personally. Your writing may be impeccable, your ideas unique–and you still receive a pass. It’s not you, it’s the industry.