On a Quora Q&A post, former Apple employee Chad Little answered the question:
How did Apple.com become the de facto spot for movie trailers? The major record companies giving up control to iTunes is a blunder I can fathom. Not sure why the Hollywood studios still debut their films on Apple.com, though.
It turns out that Apple was disgusted by a low quality Phantom Menace trailer that Lucasfilm put on their website. They contacted the Star Wars web team through DNS records and set up a meeting. Then they showed off QuickTime’s quality over RealVideo. The trailer surpassed 1 million downloads and became the most popular web video of the Internet at that time.