Why are there no real competitors to Javascript?

There is! Ones that spring to mind are Flash, ActiveX, and Java... But these all have their drawbacks. Mainly security and integration with the browser/DOM.

Flash and Java live in their own little world, by design (and to address security issues). They can't alter the HTML around them. ActiveX has access to the DOM, but also everything else on your computer.

JavaScript seems to have found a nice balance between flexibility and security, it can trivially interact and alter the pages HTML/CSS, do "safe" networking, has a decent standard library (which has things like JSON, XmlHttpRequest'sih networking, DOM manipulation, and so on). Most importantly, it's available in basically all vaguely-modern browsers, on all platforms, in a consistent manner (compared to CSS)

There are problems with JavaScript, but nothing major. The biggest is the performance. Load a comment page on Digg and watch your CPU usage. Chance are it will be 100% of one CPU core. There are projects to improve this, like SquirrelFish, TraceMonkey, and other strangely named things. But the performance is adequate to do some extremely impressive things (Google Spreadsheet, for example).

Basically, JavaScript is great, and it's drawbacks aren't nearly as big as the other competitors.

Momentum. JavaScript has been around for 15 or so years, and browser manufacturers have worked for 15 or so years to make it work in their browsers.

If a competitor came along, it would need to really bring something new to the table in order to convince everyone to a) adopt it, b) live with locking out all the users of older browsers like IE7, Firefox 3.0, Chrome 1.0 etc. and c) find replacements for all existing libraries like jQuery, prototype, extJS etc.

In short: we don't need another Standard, let's rather improve JavaScript and build on the rich foundation that already exists instead of starting back from stone age again.

JavaScript won because it was introduced by Netscape in the period when they had above 90% percent market share. IE and other browsers had no choice but to support it also.

If a new language should be introduced, it would have to be either by agreement between all major browser vendors, or in a period where a single browser have enough market share to push it through.

Microsoft could probably have pulled it off some years ago when IE has an extremely large market share (before the rise of Firefox and Safari), but they chose instead (for strategic reasons) to let the browser stagnate.

Today, a new language would require agreement between at least Mozilla, IE and Safari to gain traction, and I think that is highly unlikely. The browser vendors have invested a lot of ressources in optimization, compatibility testing and so on for JavaScript - why should they want to start from scratch with a totally new language - and have to support two languages in parallel for decades to come? The cost greatly outweights any benefits.

Anyway, it is quite unrealistic to believe that a new language designed from scratch could be significantly better than JavaScript.