Because it doesn't make any sense. The rotation in GHQ serves totally different purpose. It allows you to get different cards in the same day,
Reducing or increasing drop chance based on time is just plain stupid. I'm a programmer myself and you usually do things to improve the product, not to confuse the users, and I don't see any improvement in doing such a thing. It's basically saying "hey guys, lets fuck the users who play from hour x to y and favor those who play from y to x".