The reasons why are covered pretty well in this Mediaweek article. They speak to a bunch of advertising executives and receive great responses that indicate who gets it and who doesn't. Many, if not most of us use DVRs and when you see the picture switch to SD, you know its time to hit fast forward or pause. A couple of the executives quoted acknowledge that fact, and that an SD advertisement is basically telling the customer not to watch. HDNet founder and HD Beat friend Mark Cuban breaks it down nicely "Agency employees don't have HD at home. Where you find agencies with HDTV penetration, they push for HD. Where not, not." Additionally, producing in HD only drives up costs $10,000, which is a relatively small amount.
As evidence of someone who probably doesn't have an HDTV at home, Peter Gardiner thinks watchers aren't sitting around waiting for high-def ads. Does anyone have his email address? There are comments on a few posts he may want to read.