Some sort of technical reasons seems most likely to me for now. They probably have some crazy denormalized/multi-level-cached storage where videos can be stored

Some sort of technical reasons seems most likely to me for now. They probably have some crazy denormalized/multi-level-cached storage where videos can be stored on different media or different locations. It's possible that "changing the query" requires some involved architecture changes in that system. Why they weren't aware of this before adding the new feature is a different question.