It's also not common to need a ton of huge images all at once — in almost all applications you're either displaying thumbnails and JPEG is fine and will load faster thanks to the browser's preloader getting those requests in faster than the JavaScript can run or you're looking at something like pages where the latency is easily hidden by preloading.
That means that in most cases the main driver is lower network transfer and you need to be serving a LOT of images to outweigh the cost of having to support something new and complicated versus something as widely tested and supported as JPEG.
I wonder how good the performance of that decoder is, though; if the decoding delay is much longer that the network transfer delay, the approach becomes much less appealing.