Selling media rights is the holy grail for any writer. A big-budget movie or TV show based on your work is the fast-road to more money and wider acceptance of your work. It also can breathe new life into a piece that didn't gain a large readership on its own (like Pat Conroy's Prince of Tides) or a piece with flaws in the overall writing that hurt a series' continued readership. There is no better example of this latter case than Game of Thrones.
When I read the first Game of Thrones book almost 19 years ago, I was already a fantasy reader, but it totally changed my perspective on fantasy books and writing. I absolutely loved it. I was immediately hooked by the book's prologue. I tore through the rest of the book, and waited on pins and needles for the next one.
As each book came along, however, I noticed a huge flaw more and more: the characters all sounded the same. The fact that each new chapter shifted the point of view to another, and that the number of characters being juggled was just increasing, didn't help me either. Eventually, I got to book three, and it was there that I called it quits. I know some readers don't even make it that far.
All those flaws in the book with character voice, changing points of view, and the ensemble cast? Those issues are either irrelevant (as in character voice) or benefits in a television series. Shows that have an ensemble cast are among the most highly-rated on TV - anyone can find at least one character they love and continue to watch (this was the only reason I stuck with The L Word as long as I did) - and unless you have the same actor playing multiple roles, characters having the same voice is never an issue.
There is an added benefit to Game of Thrones becoming a TV show for George R.R. Martin: because there are actors' faces and mannerisms attached to the book characters, more people buy the books because they love the show, but they notice the aforementioned flaws less. I believe this phenomenon also explains the continued success of Harry Potter, once the film franchise started up.
So, maybe George R.R. Martin should have just pitched Games of Thrones as a TV show in the first place.