They chose a different license that made it more difficult than it should have been to integrate content from MDN. The two sites' backends were completely diff

They chose a different license that made it more difficult than it should have been to integrate content from MDN. The two sites' backends were completely different, too, so the source texts needed to be tweaked before publishing. I also remember that for for many of the docs, they favored basing them off MS's contribution (IIRC). It also had a very un-wiki-like atmosphere, where the lifecycle of a change resembled a code review, i.e., the opposite of the "be bold" policy that led to the success of Wikipedia (and the early days of developer.mozilla.org).