A lot of variability in weather. Multiple weather systems tending to come together. It's a lot harder to forecast Boston than it is Las Vegas. It also depends what you're trying to forecast. If you live in a very dry area, you're going to be right about precipitation the vast bulk of the time.

Sure. The most challenging areas to forecast are typically areas with poor data availability (Mountains) or with topological features that impact the weather significantly (Great Lakes affect winter forecasts significantly). I imagine the Rocky Mountains and Great Lakes region both have lower than average forecast accuracy.