Because they gave the defenders an indestructible "castle" and the attackers do not have the siege weapons to take the castle.
Edit: Also the fact that the defenders have infinite "supplies" while the attackers have limited supplies is the opposite of how armies are in a siege. If that was true in reality, then no fort would ever be taken.
Expecting the attackers to take over the skyscrapes with only elevators and wires as the way up is asinine.
I am theorizing that one could use the big flying transports to provide a place for the team to parachute from above, but thanks to the Nightbirds taking the same slot as the big transport, most people will take the Nightbird and leave the team without any transports.
So we need to have either more transports for the attackers, or to give an air superiority to the attackers or to introduce some Cruise missile/artillery strike that can be occasionally be used to clean up camped skyscraper tops.
As the one above me said, the lack of proper indirect fire capability is a problem