Why didn't Ymir tell Historia about Marley here?

At this point, Eren was captured, and Ymir tagged along on the condition that they take Historia with them to their homeland? Ymir told Historia that no one outside the walls would tell her that she's better off not being born. Historia responds by saying, Titans won't be saying that because they would be too busy trying to eat me.

My question is, if Ymir was trying to convince Historia to go along with them, then why didn't she tell Historia about the existence of humanity beyond the walls? Do we know why Ymir withheld that information from Historia? I imagine that Historia would be more compliant to tag along knowing that her life wouldn't be in danger from titans.