Bond yields dropped as it became more and more apparent that the United States would win the war. Then the war expenditures were seen an "investment" in a lasting and durable peace. At the end of World War II, the U.S. had 50% of the world's industrial capacity, up from 40% before the war (according to Paul Kennedy in "The Rise and Fall of the Great Powers).
The great fear was NOT that the borrowings were "too high" and could not be repaid. It was that the Axis would win and not honor the U.S. issued bonds.
Or, more likely, they might "draw" and start a Cold War that would keep bond yields high. Bond yields did, in fact rise through the 1950s (and later), as a result of the Cold War against the Soviet Union. That was a factor in the multi-decade bear market in bonds that lasted until the early 1980s.
One reason for the spike in interest rates in 1980 was the Iranian hostage crisis. Basically, an attack on the 52 American diplomats was an attack on U.S. world power and leadership, leading to a rise in yields.
Geopolitical factors sometimes "trump" economic ones.