It's not obvious that use of a pesticide would substantially harm bees, as pesticides have been in use for a very long time, and many organophosphate pesticides are fairly non-toxic to bees. Neonicotinoids, however, are extremely toxic to bees. The use of neonicotinoids is fairly recent; large-scale use only started in the late 90's, and very soon after that beekeepers started filing petitions to the EPA. They were ignored. I'd say this is more a case of systemic and deliberate ignorance/politics rather than a 'mistake'.