Manned ones? . I can think of reasons to have lots and lots of industry and science in space, but reasons to have lots of bodies up there is harder....
Uhm. Someone develops a launch mechanism that is very, very cheap for small and durable components. - Railgun-to-orbit kind of deal. This necessitates assembly in-orbit of anything that doesn't fit inside a launch-shell, but also drives down the cost of keeping labor up there alive way down because water, air and food fit just fine. Result: Big boom in space activity, lots of engineers and master craftsmen in low earth orbit building probes, space radio telescopes and so on.
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