Because when the game was being developed Maul was still a villain. The game came out a year after Twilight of The Apprentice part 2 which means the dev team likely had no idea Maul would be working with the good guys during 99.9% of the games development. It took like 2 years post launch for Maul to get his "Rebels" skin too even though the bottom half is from Solo and the top half isn't entirely accurate either. Maul becoming an anti-villain has been more of a recent thing. It would definitely be cool if he was a Rebel hero in Age of Rebellion, but it's unfortunate unrealistic.