Primarily because JavaScript was the only language provided via browsers for dynamic scripting, so they didn't have to compete or care that much about performance optimizations. Also, browsers have become bloated virtual machines that try to cater to companies that want to track things like mouse and keyboard activity, rather than user privacy and consistency for uniform experiences.

JavaScript is so popular because a majority of developers started with with web development, and a majority of the money flowing into development was/has been focused on gathering user information and pushing ads for additional revenue, not delivering quality content using a consistent interface.