I think because Ymir loved the king so much that she could continue to follow him even though she wanted to be free. Mikasa was somewhat in the same situation as she loved Eren so much that she would do anything for him. However, with Eren’s harsh words to her and the rest of them pressuring her to do the right thing; she was able to do what Ymir couldn’t. She was able to kill Eren, basically setting herself and the others free of him. Ymir couldn’t set herself free but Mikasa could. With that being said, how the heck did she know Mikasa was the one to set her free? I don’t know and neither did Eren. Was that just a quick way to wrap up some loose? Don’t know.
I think because Ymir loved the king so much that she could continue to follow him even though she wanted to be free. Mikasa was somewhat in the same situation as she loved Eren so much that she would do anything for him. However, with Eren’s harsh words to her and the rest of them pressuring her to do the right thing; she was able to do what Ymir couldn’t. She was able to kill Eren, basically setting herself and the others free of him. Ymir couldn’t set herself free but Mikasa could. With that being said, how the heck did she know Mikasa was the one to set her free? I don’t know and neither did Eren. Was that just a quick way to wrap up some loose? Don’t know.