I assume from what Purism stated in its recent article about the Librem 5 USA that another American company is fabricating the PCBs, but Purism had to buy its own solder paste printer, pick and place machine and vapor phase reflow oven to do the PCB assembly (i.e. populate the PCB with components). Plus, it had to buy some solder rework stations to fix any misplaced components. From what I understand that is a large investment in both equipment and training personnel to use the equipment. It probably costs Purism significantly more to produce the Libre 5 USA than the Librem 5.
The bigger problem is that Purism probably only has parts in stock for a couple hundred of the Librem 5 USA, which would be enough to get through a month or two of the normal Librem 5 orders, but then it would have to stop and wait until October to get more processors. I guess that would make the people happy who ordered in October and November of 2017, but it won’t change the situation for the rest of the pre-orders.
Another issue is that Purism is probably going to have a hard time getting new orders until October, and having the Librem 5 USA will probably bring in extra orders for the company from people who don’t want to wait. I don’t know what is Purism’s financial situation, but I assume that the reason why it changed its refund policy and reduced the number of developers of the Librem 5 from 15 to 10 in 2020 was because of financial constraints.