The problem with Jpgs is this:
"Forgent Sues Over JPEG Patent"
02:00 AM Apr, 24, 2004 EDT
http://www.wired.com/news/business/0,1367,63200,00.html
Forgent Networks said Friday it sued 31 major hardware and software vendors, including Dell and Apple Computers, for allegedly infringing on its claim to an algorithm used in the popular JPEG picture file format.
(snip)
The last report that I heard of this lawsuit was 2 years ago, so I don't know the current status. But given some of the software lawsuits that have occurred recently, Microsoft seems to trying to minimize their exposure to lawsuits by veering away from universal formats. (IIRC, there was a similar lawsuit about Gif usage that caused the increased usage of JPGs. In the past two years, I've seen a number of people who work with graphics drop the jpg and go over to the PNG format "just in case.")
Plus, there's still that whole "possibly grabbing a chunk of the digital camera market" thing.
Hmmm... Does Microsoft support transparency in PNG files yet? Last I heard they did not. Perhaps this new format is why they have been dragging their feet for so long on PNG.