Season eight of The Walking Dead began in October 2017. There were sixteen episodes in that season. It does seem that the reviews for The Walking Dead during season eight were somewhat positive. However, sometimes, after so many seasons, people get tired of the show and that may be why the ratings fell. Also, other shows may be showing at the same time as The Walking Dead and they have some strict competition.
It seems that the plot for the season 8 episodes were not as consistent and they moved slowly during the eighth season. When this happens, people stop watching the show and then the ratings fall. However, it does seem that the writers learned from season 8 and made season 9 better.