My understanding is that they didn't have an option. IE has so many idiosyncrasies that they couldn't make it compatible with modern Web standards and simultane

My understanding is that they didn't have an option. IE has so many idiosyncrasies that they couldn't make it compatible with modern Web standards and simultaneously retain compatibility with sites that were built for IE. Better to write a new browser so that enterprise clients can use both - one for their legacy apps and the other for new stuff and the modern web - while they get everything migrated over one at a time. Versus, changing IE in place would have forced clients to rewrite everything overnight, which would have been a much, much worse situation.