it's a pilot episode, so it's a) not necessarily going to 100% represent the cartoon's adaptation of "Act 1" (if the show even gets picked up after the pilot, which is a big if) and b) it naturally is going to include as wide a variety of elements as possible that will get people interested in the premise. Karkat is a popular character and trolls are a huge part of the story's wider appeal so it would be kind of silly for no trolls to be in the pilot at all