I'm not sure, but I remember them saying something about that. Maybe they weren't allowed to use the Big Chap for implications of eggmorphing back then, so that's why he switched to his Praetorian design. I remember them saying that Scott was having a period of not letting others use some aspects of the Alien (Big Chap) around the time of Covenant. Don't remember if this was before Covenant or probably after, when Scott still believed that a sequel was in the works.