The license is probably a big reason. Firefox is GPL, so Microsoft wouldn't be able to add any proprietary components to it.
According to https://en.wikipedia.org/wiki/Firefox, it's released under MPL. But sure it's more restrictive than most code in Chromium.
As for whether there's a lot of MPL or a lot of BSD code probably doesn't have much practical effect for compliance. In order to comply, it's easier to publish all the code than to try to do the minimum for each component on a per-license basis.