I had a neighbor/friend in admissions at UCSD. Years ago, she said they had to wait and consult with the various colleges on campus (Revelle, Muir, Marshall, Roosevelt, etc.).
It’s not just about admitting students; it’s about numbers. It’s about space, staff, construction (especially now and recently) access, housing and funding. One year they over-enrolled and couldn’t guarantee housing for freshman. UCSD is in the middle of a very elite, limited and expensive housing area. They also have staff on sabbaticals who may/may not be on campus to teach courses or to supervise GA’s.
Some departments submit their admission-related information before the deadlines and some don’t. Financial aid staff need to coordinate their numbers with admissions and with State agencies providing funding.
It’s not just about admitting students, it’s all the other druthers that go with that.