Been thinking about this. What with trying to back up and organize all our photos, screenshots, footage, and such.
Being a bit of an audio nerd, it really seems to me like JPEG is like the picture equivalent of MP3. An amazingly efficient format for the time that allowed their respective mediums to flourish on the early internet, that have become so much of a standard that they've stuck around, even after more efficient or feature-rich versions came around.
Basically anything that can play audio supports MP3, and basically anything that can show images supports JPEG. Almost every image on this site is a traditional JPEG but now I'll be thinking more about it.
The standards that come first may not be the best, but they tend to stick around.