I think there are a lot of reasons to make an Age, story/setting being one of them. Creating puzzles, creating a place to socialize, playing with ideas, etc. are all perfectly valid reasons as well. I think, honestly, about 98% of all fan-made Ages will languish - it's just the nature of things. But it doesn't mean they shouldn't be made.
As for the story of how we got from there to here, I think Trylon's idea is best - it's simple yet effective. I might modify with something like "the Bahro have never seemed interested in the surface either, so it's only those places the D'ni used that seem unsafe". That way, people can build places on Earth of they want.
From there, I'm more interested in developing the 'default setting' of the Explorer Diaspora. I think it'll give people a good place to start off with if they need a structure, or to mine for examples and ideas if they decide to write their own.