I think you're right. The maintenance to maintain them in optimal condition is high and I don't think they have the tech to do that. Of course if someone else gave them fresh ones that might be a different story. Also, they are not, if I understand correctly, truly "backpack" size but more on the order of 160 pounds or so. Of course with our Southern border in the shape it's in they could just ship the damn thing in from Mexico.
the ones we developed during the Vietnam War were "manportable" - if ya had 2 guys - and were packed in 2 major subassemblies. These were designed to take out airfields and such. Never deployed, so far as I know.