The company explains that the team working on Fallen Order was formed before Electronic Arts took over Respawn at the end of 2017. Using Frostbite was not an option at the time, as EA does not share its engine with third-party teams. The devs of Fallen Order mention that starting work on the project was connected with the need to hire many new employees, and the choice of a popular technology made the recruitment process much easier. As Vince Zampella notes, it is much easier to find people familiar with Unreal Engine than e.g. Source specialists.