I would think that would be because there are a lot of existing non-profits with .org domain names, for whom it would be far from ideal to lose that, or to have to spend larger parts of their budgets on their domain name.
... and for those that choose not to renew based on cost or ideological protest, there's the opportunity for a scammer to slide their hand quietly into the glove of trust that was lovingly crafted by the previous long-term owners of the domain.