It’s not difficult for a pressure vessel to withstand a hard vacuum, it’s difficult to achieve a very low vacuum. Lots of things offgas, lots of things are gas

It’s not difficult for a pressure vessel to withstand a hard vacuum, it’s difficult to achieve a very low vacuum. Lots of things offgas, lots of things are gas permeable enough to ruin your low vacuum, just getting equipment that can achieve a certain amount of vacuum is hard and expensive. The question is: does maintaining this require a very high vacuum to replace/maintain the working fluid. Its one of those scaling problems where you have to spend ten times as much to get to the next ten times less gas in a vessel (or thereabouts) and an absolute complete vacuum is more or less impossible.