Trivia: Mark Hamill received $1 million for his role as Luke Skywalker, despite the fact that he does not appear until the very end of the film and has no dialogue at all.
Trivia: Although Harrison Ford is first billed, he does not appear until 38 minutes into the film.
Trivia: Lor San Tekka (Max von Sydow's character at the start) is never referred to by name.
Chosen answer: The actors playing Rey, Finn, Poe, and Kylo Ren were mostly unknown, while Mark Hamill is famous worldwide as both a screen actor and voice actor. The billing order was likely based on their fame and then on their new roles. It's not a union thing, minimum wage in the Screen Actor's Guild is far lower than what they were paid. It's a combination of bigger star, better agent, and the future of the series.
Greg Dwyer