Um... because it's not simple to do such a thing. That's pretty much the whole point, and the reason you would want something with the lifting capacity of Orion... think about it. Just putting together the ISS appears to be extremely difficult for us, mainly due to the limitations of current lift capabilities (i.e. Space Shuttle can take up about 30 tons of cargo). If we were able to throw up a space station that weighed thousands of tons, in one launch, then it wouldn't need to be modular - it could be all in one block, and very capable and useful in science terms. The same goes for Mars missions - the more you can launch from the ground in one go, the easier it is to assemble a Mars mission. In fact an Orion could launch from the ground directly to Mars, with no need to stop off in Earth orbit.Besides : Why bother launching such a huge spaceship from the surface, when it's much easier to assemble it in orbit ?
It's as simple as that.
Doing it chemically, even assuming something with the lift of the Saturn V, would take several launches to orbit to put together a Mars mission...
So: in short, to assemble an Orion-type ship in orbit would be a huge undertaking, because even the smaller ones imagined would have weighed thousands of tons (the biggest single proportion of that weight, as I understand it, would have been the huge pusher plate). And it's incredibly difficult to assemble something of that size in orbit, unless you have something of the lift capability of an Orion. In which case, why are you assembling it in orbit, when you can just launch from the ground in the first place?
Having said all the above... yes, I do understand the environmental arguments against (what with all the nuclear bombs going off to get the damn thing off the ground... it really does stem from a late-'50s "hooray! wonderful nukes!" mindset). I don't see why Stephen should be condemned as a "tree hugger" for being worried about that. However, as argued by others, we'd have to look at the effects of the thousands of nuclear bomb tests in OTL, and see what the effects were, if its possible to work that out (I assume someone out there has at least tried)