Cash flow is the lifeblood of any organization. They need the cash for signing bonuses in March -- there's a reason why the season ticket renewals need to be in by then (I think). TV money gets paid year-round I believe.
It's very possible that Revis gets a $15M signing bonus and McCourty gets a $15M signing bonus for their new deals. Nobody has $30M in cash just lying around. Takes a long time to liquidate. The less season-ticket cash that needs to stay in escrow for Brady, the easier it is to pay large bonuses.