C17 doesn't require a particularly strong runway surface due to its very well designed landing gear arrangement which spreads the load much better than most other equally sized aircraft.
I believe the issue (aside from lease terms) has already been touched upon - having 4 giant FOD hoovers land on a gravel strip will hugely increase the risk of damage and therefore unserviceability and unavailability. Even Albert takes a fair old battering on gravel - regular prop changes are not unusual.