It's because they're kids, as explain by the character Barney Stinson in "How I Met Your Mother". In the episode titled “Field Trip,” Barney discovered that a woman he was seeing hated the Ewoks. This news devastated him as he believed her strong opinion against the furry little Endorians meant she was in her late 30’s, which puts her far outside his preferred age bracket for the female species. Here’s the graph he presented to explain how her hatred of Ewoks could determine her age:
By his logic, those born before 1973 were too old to appreciate the adorable teddy-bear like Ewoks when Return of the Jedi debuted, while those born after loved them. Of course, he didn’t account for the fact that the graph wouldn’t apply to someone who didn’t see Star Wars until they were older. A similar graph could be drawn up to determine age based on Jar Jar Binks appreciation.