Because as far as I am aware the amount of times a governor can ask for EM help is limited. We would 'waste' a chance to ask for a larger event or support request for this. Hence the initial idea to incorporate this into the city stone, so that it could be used continuously without using up resources. There might also be limitations for stuff to be locked down in the cities. We talked about something like that with an earlier EM (not our current one) and they indicated that individual shard-specific additions to the cities were somewhat frowned upon. Hence the suggestion to define an area in town where such things might be allowed.
But since Kyronix is apparently reading this discussion he is of course welcome to correct me