Exactly. I don’t know if this is the primary reason for closing dorms, but it makes a lot of sense. I’m a House Advisor (Resident Assistant, whatever you want to call it) and I darn well want to go home for Christmas break. To keep every dorm open would deprive dozens of students of that opportunity, not to mention prevent any sort of significant break for their supervisors, the custodians, and so forth.
It’s still this way.
Move-in was a week prior to the start of classes, it was open during Thanksgiving break (but not the dining halls), and quarter breaks are cause for lockdown.
Some stay open but food and cleaning service is suspended