There hasn't been enough vaccinated generations rotated up to this point.
Animals on the farm can breed very frequently and 'a farmer' could see 20-30 generations during his life time.
The vaccines were around (and I mean 'wide-spread' for a couple of generation only.
I think there may have been added effect, but this is not the main cause. While the effect is cumulative there can be and probably is a main reason. Many things changed since the 50th. Food, water, the way children grow up now - this may all be adding up, but adding mercury into vaccines is a crime.
Mercury was used as poison during middle ages, many of the czars and kings were poisoned by mercury. So 'humanity' has a long history poisoning people with mercury.