Probably because Black Flag is fucking revered and people would eat that shit up on the pedigree alone. It's an open world pirate game, which doesn't have much competition.
AC3 stands to gain more from a remake, but it would require some good faith from the community to buy into it, and it's not as unique of an overall experience from the initial premise compared to Black Flag which instantly forms a picture in your mind of a free booting high seas adventure with some light assassination.