Because a string is a primitive value and in JavaScript all primitive values are immutable.

ECMAScript Specification: 4.4.20 String value:
"primitive value that is a finite ordered sequence of zero or more 16-bit unsigned integer values".

So I guess the next question is "why is a string a primitive value"?

Java released shortly before JavaScript and Java's strings where immutable for security reasons, so I wouldn't be surprised if that influenced the choice. I would also imagine that immutable strings are easier to handle by the runtime in a more performant manner.

Just yesterday I wrote this comment - it touches on the impact of primitive values being immutable.