Fantasy lends itself well to trilogies and to longer series. Harry Potter was seven books long, but Warner Brothers squeezed it into eight films. A Game of Thrones is the first book in Martin's "A Song of Ice and Fire" series, which may be as long as eight books by the time it's finished. If HBO can afford the tab, it could make as many as eighty or more hour-long episodes. And if loyal fans keep coming back for more, these long shows and multi-film franchises represent a huge cash cow for both publishers and studios. The trick is finding more and more fantasy books with loyal—and large—fan bases.
Therein lies the rub. There's a reason fantasy wasn't mainstream before. It's a genre that appeals to people who play D&D and get their kicks reading about elves with names like Tanis Half-Elven and Galadriel. Unless publishers can keep finding the next big crossover, fantasy may once again return to its less mainstream, and considerably less profitable, roots. People can only take in so many teenage vampire romances and wizarding schools. It's possible that the next Harry Potter is just around the corner, of course, but it seems like no matter how many "Is Such-and-Such the Next Harry Potter?" articles I read, the books never quite gain enough momentum to go mainstream. Books like Lev Grossman's The Magicians gain wide critical acclaim, but then run into the immovable object that is the hardcore fantasy fan base.
As much as I'm enjoying the bubble, I won't care too much if it bursts. Fantasy has simply gotten better over the past decade, and most of the best titles will never be adapted into an HBO series or a movie anyways. The really good stuff these days also tends to be really edgy. R. Scott Bakker's Prince of Nothing series is so dark I'm not sure it would make an R-rating if it were translated to the silver screen. Many other contemporary fantasies are similarly adult, with lots of sex and lots of violence. Steven Erikson's Malazan books are also dark, but more problematic from a filmmaking standpoint, as the popular series spans several distinct time periods, countless perspectives, and a sprawling epic storyline. The various storylines are not obviously connected with one another even after several books. Robert Jordan's Wheel of Time series suffers from the same kind of shortcomings. What works in epic fantasy doesn't necessarily translate onto the big screen.
Plus fantasy costs too much money to produce. Dragons, spells, and fantastical worlds are expensive, even in the age of digital animation that has made this all possible. It's one thing to adapt A Game of Thrones, which is as much medieval adventure as it is high fantasy. Martin's work has little overt magic, and few magical creatures. Compare this to the work of Jordan, Erikson, or Bakker and you begin to see how studios such as HBO might be leery of the investment.
The big studios, however, shouldn't always look to new writers and new works to find their next jackpot. The Chrestomanci books by Diana Wynne Jones would be almost certain hits, in spite of (or perhaps thanks to) their similarity to the Harry Potter books. Lloyd Alexander's Prydain series would also make a wonderful series of films without stretching budgets too thin. And there are many others.
The fantasy bubble may still be a long ways from bursting, but I wouldn't expect it to last forever. For fantasy fans, this shouldn't be too demoralizing. Too much magic has always ruined good fantasy, after all.