Because it uses additional ram and adds unnecessary complexity? It's the same reason why hundreds of "Well, why can't you also do this" features get rejected. Rockbox doesn't need to do _everything_ and generally speaking you can spare more disk space than you can CPU time and RAM. That's not to say JPEG won't be implemented, just that "it's more convenient" is really the only argument for it, and "it hurts people who don't use it, even if it's a fairly small amount" is the argument against it.