IE6 was a very good browser when it came out in 2001. People forget how the landscape looked at that time: Netscape 6 was utter garbage, Firefox was still years away and Opera was a tiny, largely unknown niche browser that did cost money and was plagued by incompatible websites which were optimized for IE.

It's just that MS ceased investing more than the absolute minimum into their browser after releasing IE 6 - it took until 2009 until a MS browser passed the Acid2 compatibility test.