Adolf Hitler - It is a dark joke but... it is also something interesting (about care their bullies)

That was one third of what happened. Don't forget:
*Half the existing artists got drafted and either died or got their arms blown off
*The few that survived joined the dada movement, which basically means they decided the world didn't deserve good art

You also have to keep in mind that in this timeframe cameras have become widespread. There was no point anymore to gather your family and get a family portrait done when a photograph got do the same, better, faster and mostly cheaper.
This was the start were Artist questioned the place of photorealism in the modern era which invited experimentation which lead to the famous works of Picasso and Van Gogh and the formation of modern art as we now call it