Death anniversaries are as old as the hills. It is a prominent practice in traditional East Asian cultures, owing to ancestor worship. It is called 忌辰, 忌日 or 命日, all meaning "day of mourning". In China, such practices predate recorded history. Conversely, these cultures didn't traditionally celebrate individual birthdays; births and ages are counted as the start of the year.
Death anniversaries are also celebrated as part of Orthodox Christianity, originating from their theology.
Perhaps the most famous death anniversary is Good Friday.
The reason why death anniversaries are a big deal is the same as why anniversaries are a big deal. Years are the longest natural cycle that has significance on human life, so it's a better time than any to revisit past events.