Well, there are many reasons.
1) The film is an absolute classic, the classic of classics. Remaking something this legendary would be like remaking The Wizard of Oz or Casablanca or The Breakfast Club.
2) The acting in the film was outstanding. Also, after seeing GWTW, nobody can envision anyone but Vivien Leigh and Clark Gable playing Rhett and Scarlett. (same goes for all of the other characters). What modern actor would want to be compared to them? Nobody.
3) The 1939 movie is 4 hours long. That's a damn long movie. If you added all of the good characters from the book, it would be even longer. The book is over 1,000 pages! There's no way you can fit in everything from it into a movie of reasonable length. Also, people have shorter attention spans than they did in 1939. Nobody wants to sit through a movie that's over 2 hours.
4) Even with the "advancement in film making", there's no way you can fit everything into the plot quickly. Sure, the effects would be better (they were outstanding in the original, especially for a 1939 film) and the cameras would be crisper, but that's about it.
5) Taking GWTW as a project would be a huge commitment and stress to all of the filmmakers involved. Knowing your picture will be compared to the most popular movie of all time (literally), the highest grossing film of all time (if prices are adjusted for inflation), and what is considered "the most magnificent picture ever", is a lot to carry under your belt. Also, GWTW won 10 Oscars. A little too much to live up to, I would think.
6) David O. Selznick, Margaret Mitchell, Vivien Leigh, Clark Gable, and everyone else would be rolling in their graves.
7) Consider the few stars of the movie who are still alive, like Olivia de Havilland (Melanie) and Evelyn Keys (Suellen). How would they feel? Not very thrilled.
8) People now just aren't as interested in GWTW as people were in 1939, when it was literally a national sensation. The governor of Georgia even declared a state holiday for when the movie premiered in Atlanta! The only people who would show up would be over 50, with the occasional teenager (like me. but no matter how good it is, it would never live up to the original in my eyes).
There are so many more reasons - I could write a novel as long as GWTW about why nobody will remake it. But I'll stick to these for now.