I think it's because of her job, remember, Dearka works in ZAFT so maybe she didn't want to live there with him or something. I think they were dating or something, but there jobs, and their priorities came first. I really want to know what happened to them too, its so frustrating >.<
Also, from Gundam SEED Astray, I also like Shiho and Yzak, there cute too. I only watched a few eps of Astray though, it got a little boring after awhile ^^