What people often forget is that there was a time when IE6 was actually better than the alternatives (on Windows anyway). Around the time it was released Navigator was the main competitor and that was getting progressively less stable and failed to keep up with the way the web was evolving. And by "evolving" I don't just mean new techniques/standards/whatever - simply the way the size of things was growing as people started to commonly have access to decent bandwidth. A key example I remember well is large nested tables (semantic markup was even less easy to get right cross-browser at that point) - Navigator would sometimes spin the CPU for a full minute to render a page that IE would throw out in seconds.
This is the secondary reason IE gained share quickly (the primary one being bundling, of course). Then Netscpae fell over and it didn't have competition at all for a while so MS simply stopped trying (why work to improve when there is no competition and you can use the resources to work on something else?) so IE gained even more share on Windows (other OSs were not large enough in the desktop market to figure at that point, and mobile browsing was embryonic) without any effort from MS. It wasn't until Firefox got to the point of being enough better to attract a large share that the tables started turning and even then the change was slow. Opera was significant around the time too, but it not being free (or not free without adverts) was a sticking point that stopped it being widely adopted.
That is a key problem that hopefully can't play out the same way these days though. No one browser, even mobile safari, is so commonly used that if it fails to keep up it can't start to be ignored, and away from iDevices we don't have the pure binary choice so if B doesn't keep up with A C and D probably will and B will be forced to (or it won't matter so much if B dies).