Valid thought. Here's my guess...
The premise is based on money, of course. We want to take peeks into people's lives, but we don't want to pay for it. But then why are we willing to pay for other sorts of books?
The general sentiment I've heard from agents and editors is that memoirs from unknown people are so difficult to place because no one's going into a bookstore looking for a memoir from an unknown person. Fair enough. But why? I think it has something to do with the range of the category.
When most people think about their favorite types of books, they can tell you, "I like science fiction," or "I like historical romances," or "I like literary novels" because all of those categories have some kind of expectations... some kind of "rules." Memoir really doesn't.
You don't know if someone's going to piss you off with their religious beliefs or foul language, or whine on for six chapters about something you'd consider minor, or tell a story you can relate to in any way beyond superficially. And they don't lend themselves well to cover descriptions. Sure, you can find out that a memoir is about recovery from alcoholism or a child's death, or whatever, but what counts in the memoir is more in the writer's telling than in the plot.
So because of that, I think people much prefer to satisfy their quest to understand strangers by reading personal blogs and such, which are free and don't take up any space on the bookshelf or require any real time commitment-- if they think someone is whiny, *click!* Onto the next.
My other part of the theory is that we don't all like to admit that we like this "peeping Tom" behavior... It's like it's okay if we just watch a little reality TV about a guy trying to lose weight or read about the mom whose son is in prison on the computer, but to actually shell out money for stories like these, we're admitting that we value peeking into other people's lives, and that may even bring up guilty feelings that harken to the "never read someone's diary" rules. Even though the "diary" is clearly meant to be read, and we're being invited to read it.
Sometimes I feel like I'm being nervy when I read personal stories... like I'm a paparazzi photographer hiding out in the Dumpster behind someone's house. Like I've gone into territory I shouldn't be allowed into.
I do read memoirs-- more than a lot of people, I'd guess-- but generally on recommendation. I've rarely wandered into a bookstore and sought out the memoir section just to browse. Maybe the recommendation makes me feel like it's okay. Maybe that's the "invitation" I need.
I'm musing on all this because I actually want to read more memoirs, and I want other people to read more memoirs, because I think they're important and I hate that agents and publishers are so reluctant to take them on.
Wait, I have another theory.
Maybe it's the association people have with the word memoir. Maybe people assume that memoirs are all about sad people writing about all the horrible things that happen to them.
Maybe one day I'll write a book titled "Hi: I'm a Happy Memoir."
Okay, more thoughts, please.