Q:

Why aren't there other client-side scripting languages for websites?

Why is there just support for JavaScript and some VBScript in browsers today? I know JavaScript is good and all, but wouldn't having the option of using another programming language help promote different development styles?

A:

There is no need to add suport for multiple languages, a solution would be to standardize on a generic bytecode that could be used by language implementors. But there is currently no plans for this (it's been suggested). 
Languages can be implemented on top of Javascript too. Javascript is good enough to allow other languages to be implemented on top of it. And there are many examples of this already. 

A:

JavaScript is the de-facto standard and has been since 1996. Being a standard simply because there is no competition isn't exactly fair, but I haven't heard a lot of complaining about why there isn't another language included.
Adding another "standard" language promotes all sorts of fun little issues. 

How will they work with other languages? 
Will the DOM be shared? 
Could scripts written in both still work? 
Porting libraries to both

A:

Think of the inconsistencies between browsers for their support of javascript alone. Now think about how it would be if there were more languages.

