They will treat it like an invasion of their liberty, like some sort of socialist/libertarian push with some sort of a secret agenda.
And some more pragmatic reason: the exorbitant cost to replace the high number of vending machines nationwide so that they would take the new polymer notes.
And money interests: On the other hand... when the US introduced the new dollar coins, two senators took up the fight to ensure that dollar bills would continue to be printed, and in fact, that promotion of the new coins wouldn't be allowed to disparage paper currency. By astonishing coincidence, one of these senators was Trent Lott, who's constituents sold much of the cotton used to make make paper currency, and the other was Ted Kennedy, whose constituency included the Crane Paper Company, which had a contract to provide the actual sheets on which currency was printed.