It's a salary cap game. Your guy may be a top tier player, but the receiving team would need to not only send you the pick, but also find cap space for him at market rates.
In general, the game is not keen on sending you a lot of trade value, unless the AI team is getting back a lot of "cap value," not just overall value.
Some gamers dislike this, and would prefer a lot of trading. Unfortunately, for you/them, the game doesn't really have sliders or settings to make this your decision -- you're just stuck with a game where (like the NFL) it's pretty hard to get a lot of value by trading away the players you can't fit.