it's the lack of suitable areas to pick up water in Australia. Not many lakes in the Blue Mountains or anywhere in the Great Dividing Range where many fires are.
If you have a look at the places recently where we've had significant loss of property and/or life, there has always been water relatively close by- close enough for a CL type option to be quite viable. Lake Eildon wasn't that far from the Victorian bush fires, Warragamba for the Blue Mountains fires, lots of lakes for the central coast fires, Lake Burley Griffin for the Canberra fires although there may be a question there over depth?
Even if they take 30 minutes to do a round trip from the fire front to the water supply, the amount of water they can put on the fire is enough to alter fire behaviour. In many cases they will alter fire behaviour more in one drop than a helicopter will flying multiple smaller drops but with more water over the same time frame.
When you then have 2-3 (or more) of these things working in concert, it can be a very significant force multiplier.