IE4 introduced DHTML, i.e., the paradigm that the entire HTML document is on the DOM and you can modify it with JavaScript (and style it with CSS, which is also on the DOM).
IE5 introduced XMLHttpRequest, i.e., the foundation of the modern SPA.
Both of these were major advances in web development, and websites started requiring (or preferring) IE because they wanted to use that stuff.
From there, yeah, IE6 was mostly bugfixes and webdev languished until Chrome came around (with Firefox being great, but not enough to topple IE.)