Google's Adam Langley explains why, and this Ars Technica article adds some more context.
As Langley says:
So soft-fail revocation checks are like a seat-belt that snaps when you crash. Even though it works 99% of the time, it's worthless because it only works when you don't need it.Seems like pretty good reasoning to me.While the benefits of online revocation checking are hard to find, the costs are clear: online revocation checks are slow and compromise privacy. The median time for a successful OCSP check is ~300ms and the mean is nearly a second. This delays page loading and discourages sites from using HTTPS. They are also a privacy concern because the CA learns the IP address of users and which sites they're visiting.
