The purpose of the right against self-incrimination is to prevent forced confessions and torture. With that in mind, it makes sense to draw a line between compelling someone to provide their fingerprint or DNA and forcing someone to divulge something they know. When the state has the right to make someone talk, how does it go about it? Also, it's impossible to know whether someone actually knows the passcode to a device, so someone could end up imprisoned indefinitely just for forgetting a password.