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.