Apple is reaching more people in more ways than ever before, so like many companies, trade shows have become a very minor part of how Apple reaches its customers. The increasing popularity of Apple's Retail Stores, which more than 3.5 million people visit every week, and the Apple.com website enable Apple to directly reach more than a hundred million customers around the world in innovative new ways.
Apple has been steadily scaling back on trade shows in recent years, including NAB, Macworld New York, Macworld Tokyo and Apple Expo in Paris.
Update: Oh, and just to clarify, this is merely Apple's last MacWorld, not the last MacWorld. IDG just made a statement confirming that it's on for an Apple-free MacWorld 2010 at the Moscone Center -- January 4th through 8th if you're planning ahead.