Politics.
It is my understanding from NYPD sources that the hierarchy of the NYFD has a hang up over helicopters and the WTC.
At the time of the first attack NYPD flew a number of sorties to the roof and took off a few and landed a number [of emergency services]. There was no need for any true evacuation that time.
The result of the NYFD having their nose put out of joint was that they talked the city into sealing the rooftops of both WTC towers prior to 9-11. This was partly a result of a negative reaction to the various pro-rooftop rescue reports written by the then commander of the NYPD Air Unit William Wilkens.
At 9-11 there were no persons on the roof to rescue because they were locked down below to die in the toxic fumes - if they were lucky.
There was always a desire in NYPD to go in there on 9-11 regardless of the smoke and the rooftop obstructions and I feel sure that they would have done what they could if anyone had been seen to have battled their way through the closed doors.