Because as it stands now, UND has no authority to do what the NCAA wants it to do. The State Board of Higher Education, which governs all of North Dakota's institutions of higher learning, mandated that UND retain the Fighting Sioux nickname and logo. The NCAA says here:
In UND's case, it doesn't have any more control over its nickname than the state flag because it must follow what the SBoHE says. The State of North Dakota might want to know what business a private association has in attempting to overturn the decsion of a state government.
According to the latest polls, I'd expect that most state taxpayers wouldn't have a problem with it.