First, the government withdrew funding for pest control. Second, poison campaigns became ineffective, because the rabbits had become neophobic. Now this is interesting. By the 1980s the standard method of rabbit control was poisoning with 1080, a pesticide that is supposed to be utterly odorless and tasteless. For years it worked wonderfully, but then kill rates declined.
Zoologists now agree that what happened was a sort of high-speed evolution.
Every year the unwary rabbits were killed out, leaving those that were not necessarily more intelligent, but more shy. Some people say the rabbits are bait-shy, but that's only part of it. The zoologists studying this put golf balls out in the pastures, and the rabbits were shy of them. Millions already have been spent on rabbit-proof fencing and on killing by night shooting from ATVs, burrow gassing, trapping, and dogging. They even tried shooting from helicopters, but that may have been just showing off.