It's all about stability and predictability. Science, like other forms of business, favors an environment without sudden regulatory changes. Sudden changes make the government an unreliable partner incapable of committing to anything beyond the next elections.

If the dominant ideology changes, it should only affect future grants, not current grants. Ideally, grants that have already been submitted or are close to submission should be evaluated according to the old rules. Otherwise a lot of time and effort will go to waste.