Microsoft claimed the exploitation of this bug was "unlikely" in the wild.
Mostly because S/MIME is an essentially dead protocol, that only a handful of people have ever bothered with....
S/MIME isn't dead. It's the standard protocol to use when encrypting internet mail within a PKI. The other common mail encryption protocol is PGP, but that isn't used within a PKI. If S/MIME is not much used it's because most people don't actually bother to encrypt their mail.
I would think that Microsoft regard exploitation of this bug as "unlikely" because they don't think anyone sends mail in plain text, nowadays.