Short answer: We have tried a multiple expansion server and it did not due too well.
Long answer:
The all pop maps server only has vanilla maps because we want to have a server for new players to be able to play on and not get kicked whenever an expansion map comes on.
As for B2K, AM, and EG we are still figuring it out and things will change over the coming weeks. We may have an AM+EG server and Vanilla+B2K server in the coming weeks. We like to keep the expansion separate because at least right after the expansion is released it is the only thing people want to play hence an only EG server. Also because the expansions tend to offer different play styles such as close quarters was infantry only or armored kill with vehicle focus. Since B2K, AM, and EG have all been more balanced in their approaches to vehicles and infantry they could work better together on a single server than say CQ and AK would on the same server.
Thanks for the suggestion.