Because "20th century" counts the number of years since they started using this counting system, which is supposed to count the number of years since the birth of Christ. It was supposed to start with the year 1 being the year Christ was born. (EVERYONE agrees they got that wrong by the way - early scholarship not so good.) Before that, western culture used a system that started at the (mythical) founding of Rome.
It's 5775 in the Jewish calendar, which counts from the Founding of Solomun's Temple All of these systems just count forward from one important date. None of them says anything about counting from the beginning of the universe.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.