Actually, nuking soviets weekly was totes feasible, trust me bro

They didn't dare - not as in they were afraid of losing the war to the point Soviets would take over the US. More that they will not be capable to make a decisive breakthrough if they would attack themselves due to the Soviets having a strong army positioned in Europe and stronger airforce than the remnants of Japanese one. 1 nuke a week since when, September 1945, and that assumes each nuke reaches the target successfully? Also, good luck attacking the Soviets in winter on land. This would have required way more troops than Americans already had on the ground to counter the Soviets. Americans would be really unhappy with even larger mobilization and way more deaths coming against yesterday's ally, and European populations (except for those under the Soviet rule and maybe Germans) would be livid, expect massive resistance, maybe even armed to Allied war effort. Which makes this politically dangerous for American decision makers. It would be different if the Soviets attacked first, as the morale and sympathy of populations would be on American side then. Churchill made that plan as an exercise of what his dream scenario would need, and had to conclude it was a very risky undertaking with high chance of getting messy and really protracted. It was not going to fly anyway, the Unthinkable was a well chosen name. There were many talks in America before, during and after WW2 about wariness for "pulling chestnuts for British out of the fire", meaning doing British bidding with American resources, e.g. when British suggested to concentrate on invasion of Italy, not France. Soviets were more serious threat to British as well, hence the plan.