During the cold war, the US was always evolving and advancing their technology, especially in ECM and radar technology. The Russians did this too, but they also never threw anything away. The low frequency radars from the 50s became the stealth hunters in the 90s. The US? They dropped much of that in the 70s and 80s.....
The problem is that none of the 9-11 attackers had these "telltale factors". They were not bearded, skullcap wearing, robed folks. None of them dressed as conservative Muslims. They were dressed like regular Americans living regular lives.