Because Sam is a douchebag full of himself and his own stupid needs

The funny thing is I can totally see Sam sharing that secret with the Campbells to fulfill one of his own stupid needs. Either the need to make himself look better than Dean (because he never cracked in hell the way Dean did) or because he inexplicably wanted to ingratiate himself with the Campbells by sharing an intimacy (about someone else) with them. I'm sure Sam's sharing-with-the-family moment didn't involve disclosing any of his own dirty demon-screwing, blood-drinking secrets.