It isn't really a Triad these days. It's more like a hexaiad when you consider the various forms that nuclear weapons can take. But as to your question, silos

It isn't really a Triad these days. It's more like a hexaiad when you consider the various forms that nuclear weapons can take. But as to your question, silos benefit from very reliable coms. It's possible to communicate to subs, but it is slow and unreliable. Airpower suffers from jamming and signs of strike, since the planes need to be put in the air and that is observable. There is also a limit to the size of the missiles and therefore immediate range. If you want to strike anywhere on command with little notice, silos are best. If you want to hit foreign target with absolutely minimal response time, submarines usually win out, since most major capitals are by the ocean. But even beyond this consideration, it is good to have a diversity of options to reduce the risk of a catastrophic vulnerability existing. For example, some hypothetical weapon that could eliminate all your submarines without warning.