One possible reason for the original decline in mortality might be improving vehicle aerodynamics so far more of the little animals are lifted clear by the slipstream safely in advance of the bodywork and windscreen.
The apparent decline in mortality more generally is due to far fewer insects to meet a sticky end, by habitat loss, pesticides, etc. Not decline in farming as such, but farming methods and the loss of farmland to building developments.
If the insect populations are increasing again – and that might be patchy and according to species – it may well be due to changing farming practices now, such as less pesticide, more hedges again, wider crop-field margins, and the growing use of so-called “regenerative farming” (which appears to be a fancy word for what farmers in the past did until about the 1950s).