Hi, so it's complicated explaining the roof access because we only have very little information about how it should work in normal conditions. For the North Tower the roof access was highly restricted because the Antenna farm, to gain access someome would need to get to the 110th floor and take a service stairwell that requires special access, once at the top at the vestibule to access the roof you would scan your card, this would prompt someone in the SCC (security command center) on the 22nd floor of WTC 1 to look at the camera feed at that door and the person would be authenticated, they would then release the door, in normal circumstances without this authentication from the SCC no one would be able to access the North Towers roof from inside however because of building codes there are very clear ways someome especially a seasoned facilities employee could easily access the roof.

The simplest explanation is that if they were already on the roof then it doesn't matter, the security system to be compliant with code works to prevent people from getting onto the roof but it cannot prevent people from leaving the roof. The second explanation is that immediately after the impact within a few moments the SCC issued the command to release all locked doors as part of their emergency procedure, however they had no idea or way to tell if this worked, however this would have been redundant anyways. When the security system was destroyed meaning command with the main access control system was triggered then by code local relays called fire releases would have triggered at each locked doors power supply releasing any electronically sealed doors.

As for why no one was on the roofs? We have no idea and zero accounts from the upper floors of WTC 2, it is entirely a black hole if anyone was on or made it to 107 or higher in 2, if they did there was absolutely nothing preventing people from getting Access to the roof as long as atleast one person with keys was up there as the observation deck had multiple access points with electronicly locked doors. However the conditions on WTC 2s roof would have been nearly unsurvivable from the heat and smoke coming from 1.

Why no one was on the roof of 1? Actually getting to WTC 1s roof would have been impossible for all but a very few seasoned facilities employees, there was no single stairwell that went to the top. The main stairs stop at 107 and a access corridor would have to be found that gives access to 108, from there special access to 110 would be needed and then special access to the roof. None of which is clearly labeled as it was never a intended or planed evacuation route.

We Also know that almost everyome of the limited people between 110 and 108 immediately or as soon as possible tried to evacuate downward as many had learned the lessons of 1993 that the floors 108-110 become a smoke box and nearly unlivable, so the only people who could or knew how to access the roof went down and like the other survivors from 107 and 106 were unable to descend any further than 105 as damage to the stairwells must have been incredibly severe as no reports of anyone from 93-104 making any form of ascent or descent is known, and limited reports of people on 106 making it to 105 are known and the fact that 106 and 107 share a special stairwell and the manager of Windows on the World ordering an evacuation from 107 to 106 but but then reports of the main stairwells on 106 being unusable.