There is no good reason. It doesn’t benefit the sim to have it removed. Many of us would love to fly there in real life, but can’t, so we turn to the simulator, and now that’s being taken away from us too. Besides which, there is a very active helicopter operation that still exists at Manston, Polar Helicopters. Helicopter Training | Helicopter Hire | Kent | Polar Helicopters
The plans to bring it back to service are very real and will happen, even though there is some friction.
Take a look for yourself, TR020002-002458-7.3 - Design and Access Statement - 4 of 4.pdf (planninginspectorate.gov.uk)
If the developers don’t reinstate, when the airport does reopen in 2-3 years it’ll be too late - they will have moved on and we will have to modify the simulator ourselves. If they just do it now, we would be satisfied.
Besides, none of this is the point. The airport exists. There’s a demand to fly from there virtually. Why not support the community and actually let people fly from there?