This wasn't always the case; back in 2001, IE6 was the cutting edge. Back then, we had Netscape 4.x and few others, so IE was the best for web development. You could reference any object on the page using its ID and show/hide, apply CSS on-the-fly, and use fancy menus and DHTML.
Those days have long passed. Today, IE6 is a thorn in the side of most web developers who wish to build cross-browser, cross-platform sites and apps. A lot of people are still using it (anywhere from ~10 - 20% of surfers), so we can't drop it completely when developing for the web. However, that won't stop us from disliking, and even hating, this ancient browser.