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.
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.
It was initially called "LiveScript", but since Java applets were really popular at the time of its creation, they went with "JavaScript" instead to get attention.
As far as I know, that's because Java was the world's first programming language at the time when Netscape and Brendan Eich developed JavaScript. The story of JavaScript is certainly one of the most chaotic. Very interesting story tho
Java is not very much older than Javascript and at the time it was far from the world's first programming language. It probably was measured by hype in developer circles the Rust of the mid-nineties.