TOS only lasted three seasons on NBC, true, but then it was rerun in syndication forever, which is when it really caught on.
Granted, this is partly a generational thing. I suspect that TNG will get rebooted eventually, when the people who grew up on that show are running the studios. All you really need is one Big Name director or producer, with plenty of clout, to decide that his dream project is a new version of TNG and, voila, you have a green-light. (I believe that's basically why a new DOCTOR WHO finally happened, after the show had been off the air for a generation or so.