This sounds like a good question for Greek.
Basically a DDOS is such a problem due to the volume of incoming data and where its coming from. In today’s ograganized DDOS attacks there are often tens of thousands of zombie machines involved in the attack and its coming in from virutally every direction.
In the old days we had DOS attacks not Distributed DOS attacks so it was a matter of blocking an IP or IP range in order to blunt an attack, now that is not an effective response.