First off, the immigration system failed miserably to catch and deport those breaking the rules with reporting with their green cards and visas. Then the security at the airport was generally abysmal - not bothering to see what set off the metal detectors.
The Fire Department and Police didn't have proper radios, or couldn't use them correctly. The 911 operators were completely overwhelmed and unable to transmit information to the first responders properly. Victims were calling with explicit information that would have helped rescuers and others trapped. No rooftop rescues could be made due to intense heat and smoke. People died because they stayed with the wounded or lame, like in this story about quadriplegics trapped. So tragic.
And there are heroic stories like this this poignant article. Brian Clarke's story was on television. His co-worker was overcome by the sight of people jumping out of the North Tower and went to him for comfort. He escorted her to the ladies' room just as the second plane ripped through where they had just been standing. What is mysterious is the seemingly random choices he made that led to his escape. I say mysterious because I don't know how else to explain it. God? Providence? We don't know why some lived and others died.
I want to go back and have a do-over, with better results. Maybe the acceptance is just hard and I don't know anyone personally who died.