> The reason we don't use them is because the other options are cheaper.

Yes, that is literally what I have been saying from the beginning. Are you sure you didn't misread my original comment?

> passive radiators on the ground are orders of magnitude cheaper than on space because they can use convection and conduction.

That statement is technically correct when comparing designs to radiate an equivalent amount of heat in the two environments.

However in context (ie solar powered computing in outer space in the vicinity of the earth) it is entirely missing the point that the problem is not a lack of surface area but rather efficient heat distribution across the already existing surface area. I have no idea how much that costs in materials and workmanship but when you're boosting things into orbit I don't think the material cost of a rudimentary heat spreader is likely to be of much concern. The weight certainly will be, but you can also get away with some incredibly flimsy designs when operating in zero g.