When I did flood relief many years ago, (and on more than one occasion), ground stocks of fuel were often contaminated by flood waters, so the very first thing Ronnie RAAF would do was to fly in a supply of AVTUR for the Hueys and Kiowas in their Caribous, (in 44 gal drums), quite frequently to quite small, outlying private fields as close as possible to the action.
I can recall finding a Caribou-capable field myself on one occasion on a farm property and having to fly a Caribou pilot out to it to have him confirm that it was, in fact, suitable.
The Hercs can bring in a lot more drums in one sortie, but they cannot operate into many of the flood-affected fields, particularly gravel or grass fields when those fields are wet.
If, as someone has stated above, the Blackhawks were (still are?) limited by lack of fuel, is this another "unforeseen circumstance" (unforeseen - only by some) of the ADF no longer having a Caribou type in its inventory? Even the C-27 would be far more limited in its capabilities than the Caribou in situations like this.
Let's not even go anywhere near asking why the MRH-90s were not considered suitable for use in these floods, where large numbers of people needed to me moved.