Java applets involved running an actual JVM on every client computer, which meant basically doubling the risk surface of the browser... plus you had a completely different UI (that couldn't match the browser UI) sitting inside a web page.
I confess I wrote a bunch of Java applets back in the day; but I can't say I'm surprised they died, and I don't think they'll be back.
Heh. Nothing to confess. Before the HTML5 set of standards (circle 2008), there were certain things you just couldn't do in the browser with JavaScript. That's why there was space for plugins like Applets, Flash, ActiveX, RealPlayer, etc. I did quite a bit of Flash/AS3 (along with the Flash wasm/asm precursor named 'alchemy') development back in the day. If I was able to do it with JS/HTML/CSS I would have done that instead - but you just couldn't.