Quote Originally Posted by Qurange
It's probably more that Seifer was much easier to manipulate--he had clear things that could be twisted, and he was in the right place at the right time. He didn't have anywhere else to go, after threatening Deling.
This is pretty much what I think. Seifer was manipulated based on the views he had of himself. He had his "romantic dream" of being the dashing protector of someone. Edea was likely a figure he recognized and felt comfortable with, on some level of his mind, so he was an easy target. Cloud was always the quiet, loner type who didn't have obvious idiosyncrasies to be exploited.