(or "Eclipsed By The Need For An Ending")
The story of Eclipso begins, as do so many Silver Age stories, with a degree of necessity becoming the mother of invention. In the wake of the purge of the horror and crime comics, and the establishment of the Comics Code, storytelling in the industry was as much about what couldn't be said in comic books as anything else. With overt horror out, creators had to get...well, creative in finding ways to give kids their fix of creepy stuff.
So with werewolves outlawed by the Comics Code, writer Bob Haney had to turn to science fiction alternatives. Instead of a scratch from a cursed beast, Bruce Gordon accidentally cut himself on a mysterious black diamond. Instead of moonlight, a solar eclipse generated his transformation into a sinister villain. (And instead of cribbing from "The Wolf Man", Haney borrowed a page from "Doctor Jekyll and Mister Hyde", but that's neither here nor there.) The central concept and storytelling engine of a werewolf story remained the same, but it got the trappings of science fiction to avoid censorship.
But unfortunately, it retained all the problems of an ongoing werewolf story. Namely, that old bugaboo of all too many storytelling engines, the false status quo. Bruce Gordon had to find a cure for Eclipso, but the moment he did, the story would end. The series did manage to pull one unique trick that probably extended its life by a few years (and extended its significance in the memories of fans for much longer): After his first few appearances, Eclipso stopped being a transformed Gordon, and gained his own separate body when an eclipse occurred. This meant that Gordon stopped being a passive spectator in his own book, and became a true protagonist.
Even so, he could never really win his fight with Eclipso, only thwart his darker self's scheme of the month. And it probably didn't help too much that as a Silver Age character, the schemes Eclipso came up with (and the abilities he gained and lost with a bizarre lack of consistency over the course of the series' run) were a little zany, to say the least. Still, that probably wasn't responsible for his departure from newsstands, not when all of DC's books did the same thing.
No, it really came down to the same thing that scuttled Eclipso's 90s incarnation (in which he was revealed to be a sort of Elder God who only pretended to be Bruce Gordon's evil side for a while, as part of a greater scheme.) A struggle between two people isn't a storytelling engine, it's a story. People expect it to end, one way or another, and when neither side can ever gain an advantage, eventually audiences tune out. Eclipso might work as a villain, but he can't be the only villain.