At the time of working on income there weren't enough stations to justify the extra work it would require, not to mention getting the equations right was difficult enough.
Even now there are only about 179 stations that would qualify, compared to 13,912 facilities that can earn income. As the income code is now it's not really easy to expand it, if I rewrite it as a class it'd be fairly simple to add on a few functions to handle station income. We'd still need to replace variables in the equations with something that works in space. So for the time being I don't think it's worth the work that it'd require given everything else we need to get done. But it'll be done eventually.
No doubt that plays a role Marcus, but half of the stations that would be eligible (trading) already have their primary purpose of storing things. Leaving the three Space Colonies, which are really only there to fill the 'small, cheap, home in space' area. When we do income station I expect to see an expansion of the types. Casino, Hotel sort of thing, Medical, general purpose housing/living/repair and so on.
Providing room for space borne, cities, i like that idea quite a bit. Would be a very,very cool way to build a city as a hidden base in an empty system. And it wouldn't be overly out of hand if you can only build one station per grid location.
Casino would be a separate station like the Casino facility is separate from the Hotel facility. I'm inclined to see the Luxury as more a luxury set of flats in space than a proper hotel.
Coding stations to act like planets in that regard would require a tonne of work. Not something we have the time to do in a year where we need to finish the item functions, add repair and healing, locking rooms, hacking control of entities, atmosphere effects and everything that comes under the combat umbrella. Eventually I wouldn't mind having complex effects like that, but it's not a 'must have' thing at the moment, in my opinion.