It was because IE had a HORRENDOUSLY SLOW development cycle with minimal resources committed. It was impossible to get all of those issues fixed in a timely manner. If they can be fixed within a few months, it won't be 5+ years of entrenching people in egregious bugs before a new version comes out that breaks all of those if/elses in everyone's code.
We develop ONLY for Chrome. It (and WebKit especially) has bugs, too, and we report them regularly. But the development cycle is rapid enough that we can put a TODO in the code, file a bug against it, and a few months later we actually get to fix the code because, oh my gosh, it's fixed on Chrome stable.