The media conglomerates like to have franchises. Character stories that can run off a series of films, spin-offs, sequels, prequels and merchandising. Disney is the king of the franchise.
J.K. Rowling is the independent franchise queen. Late last week, the twitter world was a buzz over the potential that there may be another Harry Potter (Harry Potter centric) book, film or project.
The world that Rowling created is layered enough where she can explore various aspects of it with both new and old characters. Since it was a magical place, rules don't really apply. But in the case of a lasting franchise, she did make a complex set of rules, within its own language, which fans have absorbed like sun light.
LOST was never a franchise. It may have been ABC's television flagship for a couple of seasons, but it did not have the basic parts to become a spin-off, sequel, merchandising machine.
It would have been possible to have made a prequel to show the beginnings of the island, but then again "Across the Sea" was one of the least liked episodes in LOST's history. Further, in order to work, the writers would actually have to explain what the island was - - - in enough detail to hook the audience in like Rowling's wizard back alley.
It would have been possible to have made a sequel to show Hurley's guardianship of the island, but the DVD short with Hurley and Ben was a major disappointment. It showed that after the ending, the show was really over and done. For some, you can't reheat a burned steak.
However, there were a few that wanted the show to continue. They missed their characters. That is why many followed the cast members to their other TV projects (which for the most part failed after a half season). So there still is a deep connection to the show, but there is no real outlet for it to continue.
A spin-off of just the sideways world would have met with tremendous push back because fans were trained to think that the series was not about purgatory. A follow up series about purgatory would rub some people the wrong way. But then again, for this to work the writers would have to explain why the after life characters "forgot" about their most important part of their lives, the island.
When you have a final story with so many holes it could be a poster for Swiss Cheese, it is hard to imagine another LOST inspired series.