I think it's also a case of recognizability. If the Treasury / Bureau of Engraving and Printing switched to a material like plastic, I'd expect there to be a l

I think it's also a case of recognizability. If the Treasury / Bureau of Engraving and Printing switched to a material like plastic, I'd expect there to be a long period where people — particularly in far-flung places in the world — don't trust that the new currency they're given are actually US currency.