So what's wrong with HDN as a diversion airport? I have no idea if it was the planned alternate or not but SkyWest goes into HDN all the time. I also don't know exactly how the scenario played out but this is how I would assume the scenario played out because I've seen it happen before. The flight is en route, the northeast arrival gate closes up and they give you a northwest arrival and you make your way up there and then that closes up and Denver is no longer accepting arrivals. Now you're on the Northwest side of Denver with less fuel than planned and not many other destinations up there. It's been a while since I've been up there but I don't remember it being particularly challenging and they obviously have the facilities to handle the flight.