All the Greek houses at CU are off campus and always have been. The university never supervised them with the campus cops or otherwise. When the frats became unrecognized as student groups, they lost their office in the student union building, weren’t allowed to hold meetings on campus (as many groups can do), etc. Not much to give up.
The sororities continued to be recognized student groups and use the university facilities to organize Rush, can host meetings and post informational notices in the student union.