Because he'll blindly stay by her side whatever happens., But she only cause him misery, such as being forsaken by church, burned house, dead mother, if she stay with him it could be even more so she chose to lie so he distance himself willingly [tho it kinda doesn't work as we see[
I think she believes his mother's death is an excuse (rational reason) to keep roger away from her cause as long as Roger is beside her, he'll fall into problems (which she doesn't believe is done by someone like CPrince but by god's will) she's trying to finish the game (game ends when she's with one of the leads) so keeping a relationship going with Roger is just so cruel after what happened to his house & mother. She's feeling guilty for his loss, so she's trying to avoid roger with misunderstandings. As long as there's misunderstandings Roger will keep away according to her. That's what I think his arc is about.
Everyone is hurting fr and the misunderstanding is too much. I've been wondering, why won't she tell the truth to Roger. Its not her fult to begin with, right?