Well, part of it was that there weren't other browsers to worry about. At the time there was no Chrome, no Safari (no one used Macs in a corporate environment if there was), and no Firefox. All that you had was IE and Netscape, and Netscape was on kind of shaky ground at that point.
Those two kind of fought with each other by introducing new features that made web pages incompatible with the others. Also, even then corporations weren't really keen on letting you use some other browser, so IE was the go-to since it was already installed. Since the market was so slim, and seemed to be increasingly vanishing, it seemed easier to justify going with IE's extensions.
If Microsoft had kept updating IE and hadn't let it linger for years, we'd probably still be in the situation where they're in control of the de-facto standard. It's only because they sat with their thumb up their ass that things like Firefox and Chrome were able to get footholds and start disrupting the situation. Already being present and practically being named "The Internet" are really big adoption factors.