Because not all web users use the same browser, making pages comprehensible for all (or at least the most popular) browsers is an important goal for web developers. This goal is known as "cross-browser compatibility," and its long-term solution is ECMAScript, the standard for JavaScript.
Standards are very much like dictionaries in that authorities publish them, everyone is expected to follow them, and their publication is typically an affirmative response to how the technology is being used at the time of publication. However, browser implementors do not always comply with standards.
In such a case, a larger user-base makes a browser's implementation more likely to be followed by developers (allowing the browser to continue in its error). If two browsers conflict, the developer must either find a way to detect the implementation or decide what his priorities are.
When the decision must be made, most developers prioritize their user-base before the standard, so JavaScript is what the browsers say it is. However, the less compliant browsers (most notably Windows Internet Explorer) are being encouraged to fall in line by various organizations and the current cross-browser nightmare should be significantly faded by the summer of 2009 (when Internet Explorer 8 is to be released).
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