They play Champions League this year and he is their main player. With less than a year left on his contract, his price is low and no team is paying a lot when they can pay the buyout clause. Werder might get some cash if they sell him now, but they will gain even more if they qualify for CL (they play CL qualifications this August) or pass the CL groups, and without Ozil their chances are much lower to do that.
I think it was mutual agreement between Ozil and Werder. Since he refuses to sign a new contract he gets to play one more year in Bremen, for small wages may I add, make the team better with his presence, and then next year he leaves as a free agent and gets to choose his club and his wages.