Why are winter breaks so long for students?

Slate says it the result of stagflation and the energy crisis of the Carter era:

… In the 1970s, when many academic institutions found themselves in dire fiscal straits (one national task force predicted that more than one-quarter might be forced to close their doors), administrators realized that if they altered the calendar, they could reduce spending. By starting the term during late summer and by shutting their doors for a month or more over the winter holidays, they saved significantly on heating costs at a time when oil prices where cripplingly high.

Serendipitously, this cost-cutting measure coincided with a broader movement inside academia toward experimental pedagogy, like study abroad, mini-classes, and internship programs that could be completed over a slightly longer break.