An old saying that's been around probably longer than I've been alive is "Write About What You Know."
It was always regarded as good advice for new authors who may not have traveled the world, lived in a city for 10 years, or even graduated college yet. In a way, it's very true and definitely has some merit. After all, it's difficult to write about things with which you are entirely unfamiliar.
However, there is another argument to consider. These days, we're not fond of hearing both sides of a story regardless of what the subject is, because we're fixated on a "right way" and a "wrong way" and convince ourselves there's nothing in between. This isn't true.
Writing about what you know is definitely a good approach in many cases, but what about those who only know things that won't interest many people? Okay, so that's politically incorrect, but it's the truth. If the main things you are an expert on are cooking pea soup and making lace tatting, you may find there's no broad range market for your specialty.
And please, all the fans of pea soup and lace tatting, don't come after me now, I happen to like both. I've just never seen anyone write a bestseller about either one.
This is where writing only about what you know can be a double-edged sword. Sometimes, to gather a broad, general market, you have to research what you don't know and write about it.
That might be why people have difficulty selling memoirs. You're certainly writing about what you are most familiar with. (Yourself.) But, how many people are going to stand in line at a bookstore to read the life story/thoughts/feelings of someone they never heard of and don't know?
I'm certainly not saying there isn't a market for memoirs. That was not my point, before I get in trouble for that! I'm just saying that the person who wrote a great science fiction story, historical fiction novel, or rockin' children's book, etc., simply has a wider audience to play to, and therefore a higher chance of success.
So, yes, write about what you know, but don't be afraid to learn what you don't know, and by doing so, open up your potential market exponentially. Just something to think about. Carry on!