For one thing, Diageo do not seem to like their Classic Malts appearing as independent bottlings--you don't see much of the others, either. So they won't sell, or are very careful who they sell to. For another, Lagavulin has been in short supply in recent years, and there simply hasn't been much of any available to brokers--it has all gone into the OB (anything of any age, anyway).
There are lots of unspecified Islays that are supposed to be Lags. I suspect the vast majority of them are Caol Ilas. Salespersons, aware of the great popularity of Lagavulin, are perfectly happy to say ambiguous things like "could be Lagavulin" and let the public believe what it wants.