Why isn't that alternative browsers have developed then?

Alternative browsers haven't been developed? I'd say, alternative browsers have been developed ever since the early 1990s. It started with a couple of line mode browsers. Then Mosaic. From Mosaic, Netscape was developed. W3C came with Arena. Microsoft entered the market with IE. Netscape became Mozilla, which became Firefox. Opera came. Apple developed Safari. Google has Chrome. And there are numerous lesser know browsers.

Ones that could offer alternatives to JavaScript?

I guess none of the browser makers in the past saw any advantage. Perhaps I should bounce the question back: why haven't you created an alternative?

Ones that could offer alternatives to JavaScript?

But why? Don't forget, there are more than a billion users of browsers who will never ever write HTML let alone any embedded code. Most browser vendors will have a business model - they don't employ a development team just for the fun of it.

Perhaps, a Java or a Python browser.

Both done in the previous century: Hot Java and Grail. But Joe R. User just wants a browser that works with his computers, and which is preferable the same one as he uses in his office, and which his neighbour is using.

And why shouldn't he? Why would he care whether his browser is capable of running Python scripts?