Because the hijackings all happened in different ATC sectors.
There is no one controller looking at the entire sky at once. Each controller is given a designated slice of sky to watch, and he/she pays attention to just what’s inside it. Sure, there’s communication with controllers of adjacent airspace, but only what’s necessary to get the job done (and the occassional “Good morning” or “thank you”).
So when a controller notices a hijacking, he’s only paying attention to just that airplane. Actually no - he’s paying attention to the hijacking and any other airplanes under his jurisdiction still in his sector, at least until they can get someone in it help him (if they can - in some towers/TRACONs personnel is stretched a little thin). Priority (at least prior to 9/11) was with trying to keep the hijackers happy so they wouldn’t hurt someone - a strategy that works well with people wanting to go to Cuba or achieve some goal other than breaking buildings and killing people. Sure, the news is passed up the chain of command, but it wasn’t as high a priority as trying to determine 1) that there WAS a hijacking and not some sort of weird malfunction that took out the transponder and was affecting communications and 2) once it was certain there was a hijacking, trying to communicate with the guys on board to determine what they wanted and where they might be going so other airplanes could be moved out of their way.
That is the way the system was, and up to that point it worked, as far as maximixing surviors. Problem was, the rules of the game had changed and no one had toled ATC. If they had known the hijackers were planning to ram buildings notifying NORAD would have become much more important - but they didn’t know that, and had no way to know that.
So, with enough separation in space between hijackings, you wound up with multiple hijackings but no one in the system aware (at least for a time) that there were multiple hijackings.
Also keep in mind that they guys working ATC were doing just that - working. They weren’t watching CNN. At 10:30 am September 11, 2001 the guy sitting in the control tower 10 miles from my house knew less about what had happened in NYC and DC than I did - because by that time I was home watching TV, he was still at work trying to find room at the airport for yet another landing airplane. It’s not a job where you can just drop everything and run off to look at a TV set for an hour or so. He’s gotta stay focused on what he’s doing, especially in a crisis. He knew something really bad had happened, but not what, and from what the guy told me he was so busy with the “land all planes” order that he didn’t even have time to wonder about it. He just put it out of his mind and figured he’d catch up when the shift was over.
Now, I suspect NORAD is notified first thing for a suspected hijacking. Live and learn, I guess.