Here's what I know or have heard about them:
They're about a young woman and a powerful man who have a bondage/S & M relationship.
Sales of the series have gone through the roof.
The story started out as fan fiction based on Twilight.
The movie rights have been sold.
It's basically a romance.
They're not very well written.
The last two points really bug some people. If those aspects are true, what's the big deal?
Well, folks, I've been in this business long enough to know that some books just take off, and it doesn't matter if they're spectacular or flawed.
Sometimes, it only takes one unique element to lift a book to the best-seller lists. For instance, the plot in THE GIRL WITH THE DRAGON TATTOO wasn't anything that I haven't seen on Law & Order: SVU. But it had one of the most interesting and unique female characters to come along in a long time.
Sometimes savvy and/or persistent and/or effective marketing by the author or publisher or both get a book noticed. A large print run helps.
But sometimes it's just timing. A book hits the market at a certain point and touches a nerve. It gets "buzz" and word of mouth. Then the media get hold of it, and the social pundits start weighing in. People feel they have to read it, or they'll be missing something. THE DA VINCI CODE, anyone?
So I'm not getting my knickers in a twist about whether it's "fair" that a book that may be "unworthy" is a best-seller.
For whatever reason or reasons, it just is.