The main reason is that the beetle numbers are massive, especially in the hind flank. It really is a matter of just being too late to the party. It is difficult once those beetles have established themselves. You have such large populations that it becomes really difficult and unfeasible to enact control methods that will bring the populations down low enough. In those hind-flank regions in particular, the goal needs to be just mitigation.
We still want to do some control to try to minimize the risk of some spreading out of those regions, but the bulk of the work along the leading edge is where we're going to see the most tractable gains. That's where we're going to be able to suppress the beetles low enough because the densities are already low to begin with.