IIRC, they were originally planning to do exactly what you said, requiring a special tag to enable standards compliance in IE8, but there was a gigantic backlash from the web development and standards community.

Why? Because then we get nowhere; all the clueless web designers never find out about the special tag to make IE8 comply with internet standards, and continue making webpages for the broken IE rendering model for the next 10 years.

We need to make all those old websites break, because otherwise they'll never comply with modern standards. We need to have standardsbased rendering be the default because then the designers that test against IE8 will be making sites that work better with other browsers.

By forcing developers to realize that their websites are non-compliant (either from angry users or specifically forcing quirks mode) and by defaulting to standards-based rendering, we make the web design future a much nicer place to be.