I don't know the rationale. One guess is that sorting by "new" would provide more oxygen to heated commentors with a desire to express a strong opinion against

I don't know the rationale. One guess is that sorting by "new" would provide more oxygen to heated commentors with a desire to express a strong opinion against all comers. Or in general, making things a little harder is a feature not a bug that makes HN HN. I could be wrong. Good luck.