Courts (in America -- although there might be a new case about this hovering around?) basically view passwords/PINs as speech. So you have a fifth amendment right (in general) where you cannot be compelled to tell them or unlock it for them.

Biometrics are seen as physical -- like a key to a lock, or your DNA -- which you can be compelled to provide. Forcing you give them up isn't seen as incriminating yourself any more than demanding your fingerprints are.