Why did the shorts disappear during the last season of The Tracey Ullman Show?
The fourth (and final) season of the Ullman Show ran from late 1989 through early 1990. Production for the premiere season of the half-hour show had already begun in April and was running non-stop throughout the summer and fall. As attention was focused on producing the half-hour series, it was decided to discontinue producing the bumpers. The only Simpsons short shown during the fourth-season was a rerun of Simpsons Xmas on December 17, 1989, presumably to promote the half-hour pilot which aired later that same night. Interestingly enough, the bottom fell out of the Ullman Show ratings during that season and the show was cancelled.
