It's not like you drop nukes from above when you're above the target, like from an airplane, orbits don't work that way. You could fire the nukes on many missiles planned so that they would re-enter the atmosphere all at the same time for a surprise attack.
Sure, but you could only do so for a pre-planned attack at least several days in advance. That's not a very situationally flexible option, and if manoeuvring of the barrage to prepare the attack is detected, you're in big trouble. Their subs will pepper you with nukes long before your attack strikes home, and quite possibly well before you've completed the orbital adjustments of all your warheads.
Another factor is that sub based missiles can launch from relatively close to their target and just skim out of the atmosphere, well below the orbits of even fairly short lifetime low orbiting satellites. That makes for quick trajectories. A satellite based weapon up there for extended periods would need to be in a relatively high altitude to avoid orbital decay. It turns out you don't actually get much advantage in terms of strike time even for a target that happens to be at an optimal position relative to your orbit.
Finally, what do you do when the weapons reach the end of their service life? With terrestrial weapons they can be refurbished, upgraded or the expensive nuclear material recovered for remanufacturing. I know space weapons are sexy and cool, and I love me some Goldeneye style James Bond super-weapons, but they're not actually all that useful.
It's one of the reasons the Star Wars missile defence project wasn't really viable as well. At any given time at least nine tenths of your defence satellites are over Australia, Africa, the Pacific, the Antarctic, or somewhere else peripheral rather than the Russia - USA theatre.
