The reason for the introduction of this new license is because of the ambiguity of using CC-BY-SA license for code.
It is specifically mentioned in the discussion on Stack Exchange that, under the new terms attribution is required whereas under the old proposal attribution is required only upon request of the copyright holder with regard to code.
My knowledge on this subject is limited, so please correct me if I am wrong.
old proposal != old license. Previously, all content was licensed under CC-BY-SA (the "BY" part means "attribution required" and "SA" is share-alike, essentially a copyleft that's incompatible even with the GPL). They then suggested "MIT but without the attribution" which didn't go down well with some people, so now they're suggesting "MIT but you don't need to paste the license into the code", meaning that the code is MIT licensed but it's enough if you put the URL of the stackoverflow answer in a comment.
The new license (basically MIT) is a lot more permissive than the old one (CC-BY-SA), which was compatible with virtually no open source license and just doesn't make a lot of sense for code.
