> What's so difficult about optical links from deep space compared to low earth orbit, where 200 gigabit throughput has been achieved? Is it just the attenuation?

Yup; 300km vs 600000000km. Less than a trillionth of the power comes through for the same emitted power and apertures.

So, you end up having to make apertures much larger and point much, much more precisely. You can also increase laser power some, but that's a small part of your solution.

(Or, of course, you can reduce speeds to have more energy per bit).

I don't think anyone is serious about shipping back physical data from deep space, but the station wagons full of tape thought experiment is always fun.