I remember discovering these in the mid-2000s, and thinking along the same lines as the author: why hadn't they caught on and been standardised like XMLHttpRequest, etc. had been?
I think the reality is, Microsoft introduced these technologies at the tail-end of initial "dot com boom". Shortly afterward, most of the companies who might have built ambitious web apps using them went bust. Microsoft stopped investing in IE, and for quite a while afterwards, web development regressed to people building document and brochure-style sites. People were too busy trying to figure out complex CSS, and what `hasLayout` meant in IE to build component-based apps.
A few years later, web development crept out of the doldrums: JavaScript's reputation began to be rehabilitated, the term "AJAX" was coined, and companies like Google began building more ambitious web apps. But by now Firefox was the browser of choice for most developers, and had gained enough market share that you couldn't build IE-specific sites.
Mozilla teamed up with Opera and Apple, and later Google, to create what became HTML5, and that included reverse engineering and standardising a number of proprietary IE APIs. However, they chose to ignore element behaviours. Presumably because they had their own component technology, XBL, and wanted to make that the basis of web apps. And, since element behaviours never made it into Firefox, developers never started using them.
My guess is, had Firefox adopted them, they'd now be a standard part of the web platform, and we wouldn't have web components as they currently stand. There are a lot of similar "what ifs" in the history of the web. For example, what if Microsoft had adopted XBL? What if flexbox and grid had been implemented in early 2000s when they were originally designed, instead of fifteen years later? What if ES4 had been accepted and implemented? What if Cassowary had been built into CSS when it was proposed, instead of being ignored (and later used by Apple as the basis for auto layout)?