Someone who knows what they’re talking about can correct me, but I would guess that a $30 million tax write off would “earn” them $6 million given a 20% tax rate. Maybe they believe the movie would earn less than that?

Plus, there's the cost of marketing and other post-release business activity. If they shelve it now those resources are saved, or at least re-appropriated.

Even if they believed they could break even from an accounting and tax perspective, there's still opportunity cost. That is, it might make sense to quit now and move on to something with more upside.

If the movie earned $5 million they'd still get to write off $25 million, for $10 million instead of $6 million. There could be other complex subsidies involved and stuff though.