While Nathan Fillion may be saving the day on his ABC procedural comedy-drama The Rookie, the fan-favorite actor previously thrilled audiences on Castle as its eponymous, crime-fighting protagonist Richard Castle. Also airing on ABC from 2009 to 2016, Castle was tentatively slated to reach its ninth season -- only to be abruptly canceled. Here's why the network changed its mind and ended the series rather than give it a final season.
Source：https://www.cbr.com/why-castle-ended-season-8/
