The trade for Percy Harvin made his role redundant and therefore expendable.
Leon's been great for us. But he's also 30 years old, and releasing him saved Seattle $2.125 million this season alone, not including the savings next year. That is money that will be needed to help pay for Percy's salary, other free agents and draftees, as well as cap rollover to re-sign our own young core players.
As an older running back who did not have a significant role in the offense, combined with the reduced impact of kickoffs due to NFL rule changes, and a lack of breakaway speed—it's pretty clear why he was released. With two starting wide receivers in Golden Tate and Percy Harvin that can handle punt and kickoff return duties, (who are also younger, faster, and more explosive players) it vacates a valuable roster spot that Leon was holding, which could be used for more pressing needs, such as defensive lineman, nickel cornerback, playmakers, or other depth.