Why do devices like Roku or AppleTV have a particular channel and not Plex?

Some content providers will partner with hardware providers to make their content available on a wider variety of platforms. Often they do this because the closed nature of the hardware environment offers extra content security. Conversely, many content providers refuse to partner with software providers such as Plex and do not make their content available in a way which is conducive to inclusion in a Plex channel.