It's the first novel to make it to a screen. Before that, people tried to squeeze the same amount of material into 1.5 hours, or make a series where every eposode ends in the same state as when it started.
Babylon 5 was the first screen-based art to have minor and major story arcs, death of established characters (and a whole race in one instance), with a goal of having a climax and ending.
Before that, soap operas were the closest to novel-on-screen, but they weren't meant to end.
