Here in Missouri, each county is an entity unto itself, and the county road departments can do, or not do, as they see fit. So, in our county the rules of thumb they apply for unpaved county roads are: #1: Do school buses or emergency vehicles need to travel this road, or is the Average Daily Vehicle Count (ADVC) at or above "X" number of vehicles? ("X" is an arbitrary number chosen by the Road Dept. Manager) If the answer is Yes, then the county will keep the road graded and graveled. If No, then the road is allowed to return to dirt (or "mud", as the locals call them). #2: If it's a mud road, do multiple local tax-paying farmers (Amish don't count) need it for access to their properties, or do Amish live on the mud road, or does it allow access to a popular recreational area (hunting/fishing/camping etc.), or is it used by locals as a connector between two gravel or paved roads? If Yes, the county will keep the right-of-way somewhat clear and may grade it occasionally, but will not gravel it. If No, and there have been any law enforcement problems on the remote parts of the road, i.e: meth labs, drug problems, dumping/littering, excessive partying, etc., then at the point where the road no longer meets the conditions of #2, the county will vacate the right-of-way and encourage, and maybe even assist, the local farmer or landowner to gate off the old roadway.
The ones that are being closed are dead ends, loops that begin and end off of the same road, or there's a better alternate route nearby. There's an old roadway that goes across my wife's family's farm property and we can't leave an anhydrous ammonia tank cart there anymore because the damn meth-heads will steal some anhydrous and then leave the valve open and drain the whole tank.
Down south of here about 150 miles, in the Ozarks, the locals have realized the positive economic impact of having a lot of bikers, including ADV and dual sport bikers, in the area and are re-opening old logging roads and even making some new trails for us.
Hope that answers your question, although it doesn't make it any less aggravating.