Generally, because the videos could be distributed through illegal means. The rules of the section state: "In general, a clip can be posted to contribute to a discussion, for example: when someone wants to clarify a plot element.
linking/asking for video material containing the full/a large portion of the runtime is not allowed if it is distributed outside its legal means.
What does this mean? For example, linking to website containing links to full episodes or movies are not allowed allowed. Links to Toonami Jetstream or the TV-tokyo website are allowed, because they are legally distributing the video."
So the companies that own Pokemon could crack down and punish PC in some way for distributing the television show in an illegal way. It's kind of like why Youtube really doesn't have many Pokemon videos anymore of the anime show (I believe) because the means of sharing the show there were illegal, due to copyright infringement.
So that's what happened. There was an announcement when it first happened, but I can't find it now.