Mainly because the early generation ICBMs, the TITANS, ATLAS, the SS-7s were POS as far as accuracy is concerned. If you aim for the Chrysler Building in New York, you might hit the Statue of Liberty. Thus, the big throw weights to compensate. As accuracy increased, the yield was reduced. At present, we're using 60-120 kts warheads. It's a lot more complicated than that and it was luck that prevailed more than anything else. Armies on both sides of the Iron Curtain were readied, willing, and determined to fight WWIII. How we avoided it is a big mystery to me.