One other thing to take into account when talking about the shows choice of Green Lanterns is that John Stewart was in both Justice League shows, Guy Gardner was on The Brave and the Bold, and Kyle Rayner was featured in Superman:TAS. On the other hand, Rayner is far less famous than Hal and even a lot of the people who do know him are probably more familiar with him thanks to a vast horde of refrigerator related jokes than anything he actually did.
Despite being the most iconic of the Green Lanterns, Hal Jordan has primarily had cameo appearances in DC animated shows for the past two decades so while he's the best known for older fans, he's a fresh face for younger viewers.. He had appeared in an animated film or two, but that's hardly the same thing as being a cast member on a regular series.
Of course, the film using Hal was the most important thing, but it's easy to see why they would use him even if that hadn't been the case.