Why is Vikings Valhalla over already?

The series is very good, but I had hoped for a little more. For example, some contact with the original American population, or at least for him to arrive at the place he became so famous for. Even though you get to see a lot, you don't even really get to see America. But everywhere it says they don't want to make another season, but why? There's still so much more that could be done with the series, right?