Infographic: the World Series vs. the Highest Rated TV Show that Year
Reddit user joejance has combined data from two informative Wikipedia pages to create the above graphic. As he explains in the comments, comparing the Fall Classic to TV's biggest show gives a true sense of its popularity that year, without the lurking variable of general viewing trends.
1991 looks like a particularly brutal year for Boston -- both the Sox and Cheers came in second.[Reddit]