Mir and ISS are research stations for the most part, and we have much more trouble with space travel than the humans in Sins.

There could be several reasons.
Zero gravity or dangerous experiments. Testing "space technology" in actual space is also logical I would assume.
The shortened travel time might also be a reason, since even with advanced technology the energy and effort to bring something into orbit makes some things more effective if they are completely orbit bound.

Enough lore reasons for you?

Also, it waould make it easier to keep a controlled environment in space than it would on the Earth.

I can think of several reasons...

-Weapons experiment. Something goes wrong. Do you really want to UberWTF explosion/radiation on your planet?

-Space is cold, very cold, so testing out stuff in the enviorment that it will be used in, would make it ready for field use.

-Plus, there is artificial (sp) gravity. So, it's just like a planet, it's just flying.

Considering how often my planets get bombed, yes, I'd prefer my vital installations to be in space.

lol @ Brillig's comment (I agree with it too!)

Well, if you're going to test new space weapons and technology, what better place to test it out than in space.

This is pedantry, and I hope you forgive me But these research stations aren't built in outer space, as the title says - they're built in orbit. Remember that outer, in the terms of our solar system, applies to the space outside the asteroid belt. All else is inner space. I suppose where ambiguity in solar systems such as in Sins are more prevalent, it would be safe to restrict inner space to orbit, and outer space as the regions between the outer planets or something...
But orbit isn't outer space
Sorry - it's a pet peeve

But I agree with everyone else. Since things are, it seems, easier to "proof-test" in non-inertial reference frames, it would be better to do so in free-fall.

Sorry, but do the planets spin? If not, then I agree with you.
However, a question about the time scale of this game has to be asked. As an RT4X, is there any sort of accelerated timing? If that were so, I'd also agree with you.
But if it plays out in true real time, and is not a simulation for accelerated time (as are, obviously, most 4x games - note the hundred year jumps in the early game for the Civ games), then it makes perfect sense that the planets don't orbit visibly, eh?

But it's true - we need fewer planets to a star, the way it looks. BUT this is in serious danger of hijacking, so never mind

That made me laugh - thank you

military research yes, Zero G construction could require zero G research,
Civilian, mostly no.
However we are talking about a game here right?

Yes, I think research in space makes a lot of sense when you are creating prototypes for new spacecraft and asteroid mining technologies.

That was the best line i have ever heard!

I've got news for ya: You would not hear weapons, engines, explosions from other ships either. Not to mention smoke, drifting as if blown by a breeze!