The repeater tax is nasty. Right now they're staring at being about $20 million over the tax line, with a tax bill of $65 million. That'd push salaries + tax over $200 million, and Gilbert doesn't want to pay. If Shumpert stays and opts in next year, they'd be over by about the same amount (and thus have the same bill), even with Frye and Jefferson coming off the books and filling out the roster with 6 minimum salary players.
Budget contstraints are doing in the Cavs, and moving Shumpert is relatively important to fixing that. It'd save them nearly $40 million in tax bills each of the next two years. He's not worth essentially $100 million (his salary plus his marginal tax cost).