People aren't going to put things out there that they want people to see if there is the possibility that they can't see it.
Superior or not, it takes a while for these things to filter through the system. I'm only now posting PDFs using version 1.3 (Acrobat 4) rather than 1.2 (Acrobat 3) format because enough people have moved to systems that would have some form of Reader that can deal with 1.3's format (even with Reader being a free download, many people never install anything more than what came with their system when they bought it).
I think JPEG 2000 is a great format, but I won't use it to convey any images on the internet unless I know for sure that the people on the other end wouldn't have to jump through hoops to read it.
It isn't like there is any hurry, and most people these days have some form of broadband which means more and more web designers have stopped optimizing their sites anyways(sadly). Few people are as interested in size as they used to be (and still should be).