Trying to fight a DDOS attack is incredibly difficult, even with the best defenses they can cause havoc. The US Govt spends billions of dollars a month to mitigate and deal with DDOS attacks and there really good at it. SE doesn't have a war chest like that nor the resources of US Cyber Command to deal with them. There a business and there doing what they can with the help of the ISPs. Many websites deal with DDOS attacks by using massive caching servers all around the world, ala Akami. SE cant do that as you cant cache a MMO, thus they have to deal with it as best they can by filtering and blocking traffic. The ISPs will try and route you around the congestion as best they can but most of the links are going up and down due to the attack.

Here is a map of the current attacks: https://www.akamai.com/us/en/solutio...eb-monitor.jsp - Click on the Attack tab to see what is going on.
The UK, France and Germany are taking the brunt of the attack.