It may be that no one has written the channel yet. This may be true of less popular websites, since a Plex channel developer is an individual volunteer and having a personal interest in accessing the content available on a website is why those individual volunteers have usually chosen to write a Plex channel for it.
For more popular websites or online media providers, some websites limit the ability for third-party software like Plex to access to their online content. These sites have often expressed no interest in partnering with software providers and do not provide API which could facilitate individuals and companies in offering software integration with their sites. If a channel is not currently available in Plex, that content provider may not currently provide access to the media in a way that is necessary for Plex channel developers to ensure they can create a stable, working channel plugin.
Providing a consistent and stable channel is even more crucial for the more popular Plex plugins since they tend to have the largest number of users.
Due to the frequency of changes to content providers' websites, it is not uncommon for channels to "break". Whenever possible, volunteer channel developers will attempt to fix broken channels. In some instances, changes by content providers make it impractical or impossible to continue supporting a dedicated Plex channel. In those cases, the channels are removed from the Channel Directory. At this point in time, most Plex Apps do not notify users if an installed channel is removed from the channel directory and no longer supported.
For a list of popular content provider Plex plugins that are currently unavailable or problematic, please see this forum post; all reasonable efforts are made to keep it up to date.