I will respond by referencing the reasons for defining the JPEG2000 specification in the first place, which, when you read it, will explain why you don't see or hear about it:

JPEG 2000 images have a number of properties which make them very suitable for use with the Internet. Typically, Internet users are constrained from downloading large, high quality images because of their physical file size. Often providers of images must create three or more versions of an image, varying from a tiny thumbnail through to a page size image.

Digital cameras have improved in quality and resolution to a level where they are now competing effectively with traditional film. The images they generate are often no longer directly suitable for Internet deployment - the quality and size is wasted on traditional computer monitors. In part, this is because the monitor might show no more than a quarter of the captured image without scrolling, and in part because the colour fidelity of the monitor does not match that of the camera.

So, JPEG2000 was created to improve image download time, and the fact that digital images were getting larger. Now, while we still need to care about optimizing JPEG for web viewing, and something like JPEG2000 would be nice, times have changed. I think this is dated thinking, as the expansion of broadband, dynamic image loading of JPEG and auto-sizing found in sites like Flickr and Smugmug to name two, have really eliminated these issues. Rarely do you run into an issue where a DSLR JPEG is 'too big' to show on a screen. Not to mention that screens are higher resolution and much larger than they were in 1999, when the specification was proposed.

This was made a standard in 2000, and is supported by most browsers, but I suspect it isn't used because it doesn't solve any problems.

I think no one cares. Note that JPEG XR/HD Photo have been created since, and no one seems to care about them either, not even Microsoft, who hasn't updated their HD Photo spec or info since 2006.