Anyway, seriously, Mark Lawrence blogged about this recently. The problem with trilogies - especially for debut writers - is that many, many people will not buy the first two books but wait for the third to come out.
Is this for sure though? I could easily see this being a result of readers that are sick of "trilogies" by guys like GRRM and Robert Jordan... writers that initially seemed to start writing a trilogy and then drew it out to many times its original length in order to milk a cash cow. Maybe people only wait to buy book 3 because they're sick of being burned by publishers and authors that will morph a trilogy into a decology (is that a word?) for a few bucks.
Also, when publishers are increasingly unwilling to offer a cheap version, it's hard to justify taking a risk on book one of a series that may never be finished. For instance, every Jim Butcher here is no released in a new "mass market" paperback that's an inch taller than old ones and has "easier to read" paper and thus sells for $10 instead of $8. Joe Abercrombie's books all come in a massive hardcover sized "paperback" that costs $15-$20 and is unwieldy, expensive, and unnecessary. So I waited to find out if he actually finished his series before even trying any of them. Notably, all of Mark Lawrence's trilogy books are $8 paperbacks... and he was able to get his done, as a trilogy, in a timely fashion. Same for Brent Weeks. But those guys aren't as profitable as a GRRM or Rothfuss that you know can stretch a trilogy out into 6-7 books if you're strapped for cash...
On the flipside, books like Wool and Theft of Swords show that readers WILL drop money on unfinished series, even having no idea if/when they will ever end if the work is good and the price is affordable. Both of the authors of those books (Hugh Howey and Michael Sullivan respectively) began their series as standalone short novellas that they sold themselves for $5 or so a pop. Long before either series finished, readers bought so many copies that the authors were picked up by major publishers and now their books are selling well alongside the Joe Abercrombie's of the world.
So I don't think the death of trilogies has so much to do with trilogies themselves, but is rather a reaction to a publishing industry that is actively working against them. If a book costs as much as they want to charge, it should tell a whole story. And if book one becomes an unanticipated success, that should not be used as an excuse to fire a writer's editor to give them the freedom to engage in endlessly self-indulgent (but profitable) digression.
