Its time your questions are answered.
Here's a good reason - DETAILS.
There are so many details in this series. Don't get me wrong, the details are one of the many things that keeps this series so interesting and makes it so fascinating. The explanations, the connections... everything about this series makes it everything a fantasy reader would want in a fantasy novel.
That is why it becomes hard to decide whether or not this book - or series in general - should be a movie. A movie is limited to a certain amount of time, you wouldn't expect people to sit in a movie theater for hours after hours for one adaptation. Many people from the fandom would do it but there are also other people who wouldn't.
AND THE CRITICS would hate to have to sit for six hours straight - with maybe a few washroom breaks - to review one movie. Even if they loved the set design and the acting, they would still be somewhat of a downer when it would come to the length of the movie.
THIS IS THE BIGGEST reason why there has to be a TV series instead of a movie because there are so many details, so much naive-ness, so much clueless-ness for the characters for it to all be thrown into six three hours long movies. A TV series on the other hand would have seasons after seasons where each season would focus on one book and capture more details than a movie possibly could.
Another reason why is because the actors. The actors from the movie would most probably not be in a mood to do a TV series remake of the first book when the movie failed. Very logically speaking, the movie was not good. The actors were okay. One of the biggest problem most people had with the movie was that the emotions were not properly captured by the actors and capturing the emotions of the characters is the key to a successful movie.
MAJOR SCENES were not in the movie. The movie had a lot of minor scenes which were the build up for the ending yet it somehow DID NOT work out. There were maybe three scenes that were major in the movie. MAYBE. Where in a movie they could only capture a certain amount of scenes, in a TV series the producers and director will be able to capture at least one or two major scenes per episode and keep the viewers craving more.
Yes, movies make a lot of money but that's IF it is successful. A TV series has a lot of time within one season to MAKE itself successful.