ICBMs also offer a fast response capability unrivaled by any other platform. Both bombers and subs need wind up time to get to their target/get to their launch depth and fuel their missiles. ICBMs can be launched much faster. Moreover, if you look on a map of the missile fields, the silos are dispersed over hundreds of square miles, away from major cities and even the populated parts of the military bases they are on. To get the same dispersion with subs and bombers, the United States would need to build more expensive submarines and scatter our bombers around even more bases, potentially stationing nuclear weapons in countries with less-than stable allies.