Sous-Premier

Apparently the French are having a hard time translating the word "subprime" into their language. While some news reporters are using the English word when discussing the mortgage problems of the US, the government says they should use the phrase "prets hypothecaires a haut risque," which means "high-risk mortgage loans."

It's not as if "subprime" by itself makes much sense in English, so I don't know why they're reluctant to use a similarly nonsensical, yet easy, phrase, like "sous-premier." It has the same Latin roots. Why not?