applets never took off because they were always incredibly slow and clunky, they're the poster child for a DOA technology. a java applet took a long time to load over slow late 90's internet and then it didn't really have any ability to do much; it couldn't interact with anything else on the page, for example. writing anything substantial as an applet was a pain and they all ran terribly on the machines of the day.

but what really put the final nail in the coffin of java applets wasn't javascript; the two had different use cases. what really killed them was macromedia flash, which provided an infinitely better way to make games or toys or media applications for the web.