The introduction of foxes in order to control, or even decline, the rapidly growing rabbit population was not effective due to very simple reasons, reproduction rates and available area for living.
The rabbits are reproducing quicker than the foxes, and not just quicker, but in larger numbers with every lither, thus outnumbering the foxes by far, even if each fox manages to kill a rabbit or two each day.
The area available for the rabbits to live and spread out is huge, so they are not limited by a small amount of area where their number will have to stop to grow because of lack of food sources, thus spreading around without any problem, especially because of the lack of competition.
