Because if you want to "win" arguments by randomly swinging hundreds of years to make a point, then it's pointless, because anyone can pick a point in thousands of years of History to show "look - they were bad here".
I think discussions about modern history are sufficient for the post-WW2 period, as there was a global consensus on international law and the Charter of the UN.
If you hold grievances about events hundreds of years old to make points about current events, then it's pointless.