There are many pluses for living off campus junior year. The students learn a lot about life when they experience all that goes with it (independence, landlords, paying utilities, roommates leaving the heat on during the day, etc).
Many who do a junior semester abroad will work with fellow students/friends to split an off campus arrangement for the year. E.G, one is abroad in the fall while the other is in the off campus housing, and then they swap in the spring.
They return to campus senior year because for seniors, “on campus” is the place to be. Just spend a Saturday AM in the mods before a football game and you’ll easily understand why.