That's easy: there are too darned many of us.
I just returned from Magna Cum Murder, which is a small con. Attending cons, while a lot of fun, always underscores how many people are out there writing mysteries--and a lot of them a pretty good at it.
One man at an informal discussion said it out loud, "I know I'm as good as a lot of the best-selling authors, but I just can't get recognized."
You get good--even great--reviews. You get nominated for awards. You splash your book covers all over the Internet, the local bookstore, wherever. People tell you how much they loved the latest book.
But there are hundreds of authors with similar stories.
Now, some fudge a little, saying they're "best-selling" authors when they mean that one day they hit the top ten of a very tiny slice of the Amazon pie. From what I hear, you can manipulate your subgenre to the point that you are in the top sellers for that subgenre because there might only be a few dozen books listed. There are other ways to do that, too. For example, I could call myself a best-selling author because for one week in the summer of 2010, my local bookstore sold more of my historical mysteries than any other. They posted that on their site, so I could...well, you get the idea.
Anyway, the trouble with being a mystery writer is that most of us are cognizant of the fact that there are lots of similar books written by lots of similar writers. It's hard if you have any sort of modesty to scream that your books are the best ever.
That doesn't keep some from doing it.
For most of us, the hard-sell just isn't a comfortable way to go. We might feel--maybe know--that we're as good as most, better than some, but that just isn't going to get much attention. Modesty is a wonderful human characteristic. For an author, it is probably a predictor of anonymity.