Today, the advantages of PCC there have not been established, in ways that the real world cares about. Digital signatures seem to work fine in the mass market. The device driver example may be a good one. Microsoft wants high-assurance Windows boxes that use PCC device drivers. If MS thought they could get away with it, they would require PCC tomorrow for device drivers.
OS crashes because of bad drivers really hurt their credibility.
Win2k now has signed device drivers. But getting a signature costs a lot of money because one or two MS engineers have to sit down with the code and go over it with a fine tooth comb. Not many drivers will be signed in this way. More powerful specification mechanisms are needed. Having specified and built a checker, you still need to get the proofs from somewhere.