At the time the Soviets were massivley outgunned on the ICBM front. They only had a handfull of SS7's/8's(maybe a couple dozen) and they were liquid fueled and took a long time to prepare for launch. This is why they needed Cuba to get their shorter range IRMB's within reach of the US.
The Soviets still had a large bomber force but few if any SLBM submarines and those that had SLBM's were limited in number(3 each i think) and had to surface to launch because the missiles were in the sail.
The Soviets backed down because the US caught them before they were ready and had no choice. It may have turned out different though if the Soviets had another 3-4 days to get all the missile regiments ready as only 1 I believe was operational at the time of the UN slam dunk by
Adlai Stevenson.