The USPs are still made in Germany unlike the P, VP, and HK45. The USPs are more expensive than the latter now. With the P, VP, and HK45 being so cheap now I think the USPs just aren't as plentiful.
The HK45’s are the only ones in that list that have been US made. All P30’s and VP’s are completely German made, they just dropped the price point of the P30’s to basically match the VP’s.