1. There is no moongate directly into Brit, the nearest gate requires one run or walk into the city center. Luna is just a second away from any moongate or crystal teleporter.
2. There is no space available for a central shopping area in Brit. Luna has provided that, and people are making a great deal of gold (which can easily be converted to cash) because of the ease of placing vendors in a central area, effectively creating a massive shopping center.
3. For some reason, EA chose to "forget" about the ongoing invasion of Brit until people were forced to use Luna. And for some reason, after one or two half-hearted attempts to have mini invasions in Luna, even those were stopped.
As was mentioned earlier in another post, there were hints from time to time of burning luna, perhaps just the walls.
If nothing else, limit the number of characters within the central bank area in luna..... give outside access to the shops maybe?
*hint*
*hint*