I read that nearly 60% of the incumbent Sheriffs run unopposed. Why is that?
Often you have to be a certified law enforcement officer to run for the position.
Many counties in the U.S. are rural and have very low populations, with a small pool of potential candidates.
Once someone wins an election once, they have a great incumbency edge in subsequent elections, over the other viable candidates, who are usually their subordinates or former subordinates.
The incumbency advantage is heightened because elected sheriffs are usually a partisan political office. There is a strong norm in both major political parties to not remove an incumbent in a primary election unless the incumbent have grievously screwed up. And, most rural counties are also very solidly leaning towards one political party or the other (usually the Republican party), so the primary election is usually the only election that matters in those countries.
Also, I wouldn't be surprised if "unopposed" means lacking an opponent in the general election, rather than lacking a primary challenger. In that case, since most rural counties have a dominant political party, running against an incumbent of the dominant political party is usually going to be futile. And, in a rural county, it doesn't pay to enter into a conflict like a contested election, when you know you can't win. That can have long term negative social and professional consequences for you in a way that it doesn't in a more urban area.