This is a recurring story with Barnes. I'm not going to call him a or a cancer, but this is third consecutive summer that his most recent employer decided not to retain his services despite a reasonably productive tenure with each of those teams.
Summer of 2008: After his second season with the Warriors (the only team to ever retain him for a second season), they didn't offer him a contract and he eventually signed with Phoenix for the minimum.
Summer of 2009: After starting half the season, Phoenix was not interested in keeping him and he signed for 1.6M with Orlando.
Summer of 2010: After starting 58 games for Orlando, Barnes opts out in hopes of getting a better deal. Orlando never makes an offer and signs QRich instead. Swingmen like Morrow, Wright, Bell, and Brewer have signed deals for 3yrs/10M+ and Barnes is still on the market.
I won't pretend to know the exact reasons why Barnes has repeatedly been spurned by the teams that know him best, but clearly reasons exist.