In my opinion, Gaddis presents two main arguments as to why the Cold War did not ever become a hot war. One is the polarity of the international system during that time. The other is the relative isolation of the two countries.
Gaddis argues that the world was bipolar during the Cold War. He says that this helped bring stability because there were so many fewer opportunities for conflict to arise (unlike in WWI, for example, where there were all these powerful countries with changing and complicated relations).
Second, Gaddis says that the US and USSR were less likely to go to war because they had so little to do with one another. There were not constant interactions that could lead to conflict.