Because the company never earned any money from the movie. The whole notion of a "tax write-off" is misunderstood, in part because journalists mention it casually and inaccurately. The company isn't "making" money by not doing anything with the movie. They are just taking the deductions attendant to creating the film, but then not bringing in any revenue along with it. If you lose money, you get to save on taxes.
In rare circumstances, it can be advantageous to make slightly less money so that you don't trigger some tax disadvantage (e.g., phase-out of some benefit). But in general, it's better to make more money and pay taxes on it than to not make money at all. From reading the articles linked here, it seems like what happened was that there's a new executive team, and they decided not to release a film that was green-lit by the previous executives. Not really about taxes at all, IMO (I am a former tax lawyer, FWIW).