Netscape had market dominance at the time so Internet Explorer had to keep bug-for-bug compatibility with Netscape to keep up. Given the amount of bugs and the lack of documentation of JS, this must have been quite frustrating for MS who was used to calling the shots.

> Bill Gates was bitching about us changing JS all the time

Emphasis mine

MS had licensed JavaScript – and, of course, their implementation was incompatible (at least in certain parts).