A few people commented that if Ty Harden really wanted to play for Seattle, he could just request a trade instead of taking a year off. Apparently, that's what Wilman Conde did, but the Chicago Fire haven't let him go. There are probably all sorts of reasons. For example, what organization wants to strengthen a conference rival, especially when they barely made the playoffs last year? Suffice it to say, it's not as simple as "Ask and you shall receive".
By the way, I'm not saying it should be, either. It's true that many players are accomodated, but it almost always is when it doesn't inconvenience the trading team very much, and it rarely happens intraconference.