The TAF that the crew had prior to departure didn't require an alternate, unfortunately for the crew, it seems that they didn't get the revised TAF issued during their flight. How ever they were made aware of the situation and told the weather is now below minima on descent. If you look at flightaware, you would notice that the aircraft leveled out at F170, climbed to F250 and diverted along with a QF737. NO CREW would ever divert away from a major city airfield without first crunching the numbers and deciding that a diversion is the best option.
As a turboprop or jet climbs, they burn a lot of extra fuel, and so once the decision is made to divert, there is generally no going back. Especially if you weren't carry alternate fuel to begin with. If you divert while still in the cruise, you have a much longer range as no fuel has been burned in an approach/missed approach and climb. More range+ more options....
Well done to the crew (and all their support staff) for getting the aircraft on the ground safely!