I have a completely different speculation than JamesCB.
I think half the commercial directors are wannabe movie directors, and are going to shoot their commercial in film and widescreen (or HD digital video) because they are artistes
I think there's a film master of many of these commercials, and probably a high-def master of a lot of them.
(Just listen to the soundtracks of a lot of these commercials. The production quality frequently exceeds the main program!)
I think the ad agencies know that high-def is going to be the best showcase for their products and want to be ready as soon as they can get the high-def transmissions.
The 2003 Super Bowl was the first I'd seen in high-def, and I noticed that all the movie commercials were aired in high-def during the game (with terrible audio level problems).
Like the author of this thread, it continues to amaze me that these agencies and directors aren't getting their beautiful mini-movies on the air in high definition, and instead we get letterboxed SD commercials.
The networks and stations must have some serious road blocks in place, technical or business-wise.