My take:
The religious ground from which the WT took root and initially grew was the US Protestant base, the same base from which the Adventists arose in the 1800's. There was a strong anti-Catholic rhetoric which was part of the culture. The hatred of Catholicism continued through the Rutherford years; Rutherford's bashing of Catholics and his public jabs with Father Coughlin were well known in the '30's. The anti-Catholicism diminished as they started to expand more outside the US, especially to the historically Catholic countries of the rest of the Americas. Nowadays they have to craft their message more carefully to appeal to other cultures, especially as the majority of JWs are not in the US anymore.