To that Vect would not be who he is today without the fall. He gathered up refuges and fled to Commorragh. Their he took out the last remnants of the old Empire by ending the rule of the Noble Houses. Physically they are different in that they are no a psychic race and instead get their power from pain. Socially they are different because they don't follow "Rules" of the old Empire.
Its the same as the United States being different then the old British Empire. Or any country that changed after it revolted from the previous regime. The new one is not the old one.
So. The Necrons were given credit for an important part of the setting and the DE were not. That's the whole point. Shoehorned in or not the Necron fluff is tired to important parts the setting. The same could have been done for the DE but was not.Originally Posted by Lord Zarkov
When the DE were fleshed out. They made them a minor faction with huge potential. But until they actually do something significant to the setting, they will remain a minor faction.
To touch on the Necrons being "Older". I played against more then a few "Dark Eldar" armies long before the Necron became a race. The difference was that back then the current idea of the Dark Eldar had not been locked down. So they generally were just dark painted Eldar Armies with a Slaaneshyness to them. Keeper of Secrets, used as an Avatar for example. So the ideas for DE existed long before the release of their first codex. Their current fluff was just one direction they could have taken.