Also, this flight was an overnight red eye flight to Bejing, so most of the passengers were probably asleep or dozing already.
Less resistance so they won’t fight back like flight 93.
No resistance because nobody knew that anything was wrong. They all just slipped into unconsciousness and then death silently and unaware of what was happening.
Just like in 1999, when the Lear Jet carrying Payne Stewart lost cabin pressure. All on board died while the plane was in the air and continued to fly until it ran out of fuel. Just a few seconds delay putting on your oxygen mask is enough to finish you off.
They didn't even realize it. It happens so suddenly and you are unaware. The best way to silence all on board so you could steal an aircraft. This also explains why their phones continued to ring and not get answered.