It was partly marketing, part to appease Sun Microsystems at the time, whose Java Applet product were supposed to be the true unifying web platform. Having a built-in scripting language in the browser annoyed a great many important people, who felt it undercut the importance of Java. Calling it Javascript gave the illusion that it was a smaller subset of Java, (even though it clearly isn't), while also benefiting from the more recognizable/marketable name 'Java', which was the new hottness.

[–] drcobaltjedi@programming.dev 7 points 2 years ago

I actually had a boss who thought they could let me do java at work (can't in house executables were a "security risk" according to corprate) but he ended up making me do javascript, which I don't know. I got very frustrated trying to get him to understand JS isn't java and now because of JS's weird behaviors I can't actually do the thing he wants so easily.