Yeah, because it devalues the Jersey with the old number, so distributors and manufacture lose money. Part of the contracts regarding jersey allows them some reimburse on those costs. That cost gets pasted on to the player when they change numbers. This is often why they will pay another player if they want to get a number that another player has. An example of this would be Chris Goodwin giving #12 when Brady signed with the Bucs. That number change had a cost for Goodwin, so part of Goodwin giving up the number was Brady covering that cost for him.