The show wasn't that well received by critics, it seems. Big schism between Rotten Tomatoes and IMDB. I haven't read the reviews, but I'd speculate that's because most critics nowadays are members of the social justice left. They review the politics of shows/movies as much as they review the aesthetics of them. In this case, the show was about a white Marine exacting vigilante justice with guns. Throw in the fact that it treats the anti-gun control argument and other right wing positions with fairness or at least with humanity, and it's not going to be popular with the left right now.
Most people don't give a shit about that stuff when they are watching a TV show though. Hence the high IMDB rating.