I agree that it may be easy to switch service providers from a technical front, just plug in a new cable, but...
Perhaps it's more as a means of protecting contracts? When Gatwick was lost, I imagine the incoming provider was relying on a certain percentage of incumbent controllers staying put due to family/life ties to the area. If your controllers are 'remote' there may be less likelihood of them wanting to remain in situ and be transferred to new employer, as they are no longer bound to the airport area, so unless the new provider intends to set up their remote kit near the current one (but then why not?) there may be less numbers looking to stay on, and so more numbers needed from the new contractor to find from 'scratch', so perhaps a less attractive bid to win...