One PKI vendor employee confided in us a few years ago that they had great success selling their PKI solution, but that customers were still unhappy. That requires a massive change in the underlying system software. Under SSO, you come into work in the morning, plug in your smart-card, enter the PIN that activates it, and for the rest of the day, you don't have to do any more logins. All of that is handled for you by the SSO mechanism. Of course, it's attractive.
Authentication is a pain.
Anything we can do to avoid it, we'll jump at. Authentication is supposed to prove that the user is present at the controlling computer, at the time of the test. Under SSO, when the user has to rush to the washroom, any passing person can walk up to that user's computer and sign on someplace via the SSO mechanism. We are not that cynical. Our assessment is that security is very difficult, both to understand and to implement.