ECMAScript 4 had to be dropped when people realized that turning JavaScript into a JS-Java hybrid increased complexity rather than decreasing it. But the process of drafting ES4 and then disputing whether to go forward took a lot of time.
And don't forget that IE6 was Microsoft's latest and greatest for a long time, with little prospect of an upgrade. When IE progress was frozen with huge market share, the prospect of updating JS seemed like a pipe dream.
>ECMAScript 4 had to be dropped when people realized that turning JavaScript into a JS-Java hybrid increased complexity rather than decreasing it.
AS3 (which completely conforms with the ES4 draft specification) is a lot nicer to use than JavaScript (ES5.1 included). It gives you means to organize your code and your tools also have a clue what you're doing.
And don't forget that IE6 was Microsoft's latest and greatest for a long time, with little prospect of an upgrade. When IE progress was frozen with huge market share, the prospect of updating JS seemed like a pipe dream.