Why? Because the lamp was yellow. Up until relatively recently, Green Lantern power rings would not work on anything yellow. The weakness was a revision of the original Green Lantern concept, the one introduced in 1940’s All-American Comics #16, by Bill Finger and Martin Nodell. Even though the first Green Lantern Alan Scott was not part of the Green Lantern Corps and had abilities based in magic instead of (very comic-booky) science, his ring had a weakness. It would not work on anything made of wood, leading to similarly embarrassing moments when he gets knocked out by a gangster with a lamp.
Of course, both the weaknesses to wood and yellow were initially little more than genre conventions, an excuse to create tension in a story (fun fact: the wood weakness actually predates the introduction of Kryptonite, which isn’t mentioned until a 1943 episode of the radio show, The Adventures of Superman). But over the years, writers began to give more reasons for the weakness. First, it was just that yellow was the color of fear; later, it was a necessary impurity, some aspect of the power harnessed by the Guardians that could not be avoided.
However, the most significant explanation came from writer Geoff Johns in the 2002 series Green Lantern: Rebirth. Set after Hal Jordan had become the villainous Parallax, and after he heroically sacrificed himself to reignite the sun, and after he sought redemption as the ghostly superhero the Spectre, Rebirth had to both revive Jordan and to let him off the hook for everything he did as Parallax.
Johns’s solution was to reveal that Parallax was not just a scary-sounding name that Hal picked for himself after his heel-turn. Rather, Parallax was a cosmic entity that embodied the emotion of fear. And, wouldn’t you know it, Parallax is yellow.
Long ago, the Guardians secretly captured Parallax within the Central Power Battery that gave all of the rings their power. Thus, it was Parallax’s presence that made their rings useless against yellow. When the Green Lantern Corps executed Sinestro in 1988’s Green Lantern Corps #222, his consciousness was actually stored in the Central Battery. There, Sinestro discovered Parallax and slowly started leaking his presence into Hal via the ring, until the point that Parallax could completely possess Hal. And also make his hair turn grey, for some reason.
Of course, Rebirth ends with Hal completely free of Parallax. And, just as important, free of grey hair. With Parallax out of the Central Battery, the Green Lantern rings now work on yellow. Sometimes, the ring user still feels something akin to fear when they encounter a yellow object, requiring them to overcome that fear to deal with the object. But for the past twenty years, yellow hasn’t posed a problem to members of the Green Lantern Corps.
