There could be many reasons why they're not showing certain aspects of the game. But...
Watching this video between about 40-56 seconds and seeing the drastic popping in and out of ground details, I can understand why it might not be a good idea at this point to show London city, if huge swathes of it would be appearing and disappearing in a similar way. Better to emphasize the positives in your product or project, especially when knowing there'll be no shortage of people ready and willing to emphasize, or even over-emphasize, any perceived negatives or failings in it.