For example, when requesting ATIS from an airport, it will first try VHF, and if that fails, it will try HF. And if all hope seems lost, it will try Satcom. Why is this?
The main concern is that radio spectrum is a shared resource, meaning that while you’re using it, nobody else within the range of your transmission can use it too. That means you always want to use a shorter-range band whenever possible.
Another is that HF channels are significantly narrower (lower data rate) than VHF ones, so not only are you consuming a more scarce resource, you’re also consuming it for longer periods.