Probably it was just a matter of timing and circumstance that Babylon 5 never made it to the big screen. It had TV movies but that apparently wasn't enough to keep it going. I have seen a lot of Star War and Star Trek memorabilia but I don't recall any Babylon 5 memorabilia, but I must have seen it, having gone to a lot of shows. Without the big screen movie exposure, Babylon 5 was globally distributed on VHS, which was circling the drain. It would still be a few years before DVDs became cheap, and the internet was just beginning to make things immortal. Maybe because Star Trek was born on TV, it had to fight its way to achieve eternal existence, while Stars Wars, born on the big screen, never had to look back.
When a name doesn't stick in the public's mind, it doesn't work. Any number of popular works can be attached to the name, but if the public doesn't endorse it, the name doesn't get used. Some eco science fiction stories and movies were big in the 1970s, but they were only seen as science fiction, and even now, trying to call them eco science fiction is a futile exercise. Maybe the words science and fantasy can't be seen together. Maybe it encompasses too much territory so it is hard to imagine or it arrived on the scene too late. For writers it is easy to understand, but for readers it might be like saying lets replace science fiction with a new word, science fantasy. From my understanding, science fantasy is bigger than science fiction. I know people who call Star Trek fantasy because they don't read science fiction.