I've heard it explained that "hey" used to be more of an urgent way to get someone's attention, rather than a casual "hello" like it is now, so it sounded rude to some older folks.
Oh wow, so really we were giving them a mild heart attack, every time we were saying hi? I don't know if that really justified the grammar police (not using that other word casually, anymore) but also these are the people who created boomers, so they definitely had to have some issues they were never able to work through.