Excluding Alan Scott (who was never a member of the Corps and whose powers are magically based), Hal Jordan was the first Green Lantern, and he was the only GL for a significant portion of the title's history (at least from Earth). Telling his backstory is easy and doesn't have to include a lot of other characters.
Guy, historically, hasn't been as popular as Hal. He also has a lot of background surrounding him, and has had radically different portrayals - He had a gold ring, he was a GL, he was an alien, and he has sported a bowl cut. He's difficult to fit into a solo starring role.
Kyle ALSO has a lot of difficulties in being used as a GL in a stand-alone story: Telling his background requires telling the background of Jordan, Jordan's fall to Parallax, the destruction of the Corps, etc. It's a lot of backstory to squeeze in before you can tell a story about him (unless the story IS his backstory, in which case it must cast Hal as a villain).
John Stewart is easier, in some ways - he has a relatively isolated backstory. He was a soldier, and is an engineer. His secondary characters tend to be able to exist without drawing in too many outside stories. The downside is that he's African-American: studios tend to prefer white characters to star in their films. John Stewart was GL for Justice League, yes, but would he have been if it were just a GL animated show? He filled an important role on the team by being black - WW and Hawkgirl didn't understand a lot of things about humanity (or 'Man's World') and having him on the team allowed the writers to explore race relations easily. He also was a better straight man for Flash, while Hal (at least in his younger years) would have competed with Flash for the role of comic character.
In short, Hal is the most recognizable GL (though that may change as people who grew up watching Justice League age), has the least amount of canon baggage brought into the story with him, and has a character that has been relatively stable, personality-wise. The other GLs don't meet all of the above criteria.