Keeping it secret ensures that they don’t have to deal with all the characters constantly attacking them to return them to FTL somehow. If they think that FTL doesn’t exist, they’re forced to go about their lives, and make peace with their situation. Plus from a writer’s standpoint, it makes much more sense to go this route, so that you can have a status quo that is easy to keep track of going forwards. Don’t want TOO many story elements that they have to juggle in the air, it makes it harder to tell quality stories when they have so much to keep in mind.