Java applets was never particularly popular to begin with despite all the applet books. Back in the really old days:
1. Microsoft competed with it in every way possible (ActiveX, J++)
2. The browser could crash from just the applet runtime loading up, which wasn't uncommon when the machine had 16 megs of RAM and a few megs left on the 1 gig hard drive. E-commerce and serious sites are very paranoid of getting in every customer possible so you wouldn't find any applets in use here.
My personal opinion:
1. Mobile devices killed off applets of every type as excess baggage. Hundreds of megs have been saved.
2. Applets of every kind were designer unfriendly compared to CSS making division of labour harder, Java was particularly vicious in the ugly department with the gray square it would leave there while things were loading up, and the AWT made everything looked like a 1970s unix wm inside that box.
JavaScript didn't so much succeeded as it just survived from being integral to modern web page rendering.
>>5
LiveConnect, but I think JavaScript is an required intermediary to the DOM.