Cosby never accepted an Emmy nomination.
https://books.google.com/books?id=prQDAAAAMBAJ&pg=PA57&lpg=PA57&dq=the+cosby+show+emmy+awards+actor+competition&source=bl&ots=l3o1SxqxS5&sig=-E-dYWOqYAHRjaqDmpSwgAKgMgU&hl=en&sa=X&ved=0CCgQ6AEwBGoVChMIqrreo5yZxwIVwiuICh0w4AhM#v=onepage&q=the%20cosby%20show%20emmy%20awards%20actor%20competition&f=false
This explains the supposed snub of his performance for an Emmy nomination for the show. However, despite his "longstanding personal preference", he still accepted those 3 consecutive Emmys for I Spy, 2 decades before The Cosby Show. What made him change his mind?
A cross between Star Wars and the smell of ass! - Jon Stewartshare