We can discuss the details of the engineering design processes for this SF dip if you wish, but it will be a somewhat technical conversation. In summary, it was not malicious, but it was a result of what were known as the best engineering practices at the time and the limitations of the tools and information available to the designers.
I've not suggested that it was malicious, I think it was done out of a cost-
consciousness inherent in the undertaking of a large-scale project such as the Interstate Highway System. I would also expect that the design loadings of interstate bridges was lower than the actual loadings they experience. Metal Fatigue, while known, was not fully understood at that time (if it can be said to be fully understood today). Material quality control probably wasn't as good as it is today, either. It is quite simple to get by with substandard materials in a bridge that designed with a safety factor of 2, harder with a safety factor of 1.5, and probably impossible with a safety factor of 1.2. Again, I'm not sure of the criteria used in design of the interestate highway system, but a look at the design of a 4-lane interstate bridge compared to a 2 lane bridge of turn-of-the-centrury design reveals a significant difference in design, tilting (again in my unqualified opinion) in favour of the turn-of-the-century design being superior. Not to mention that they are generally more aesthetically pleasing. Look to the Brooklyn Bridge as an example, or to any of Brunel's bridges in the United Kingdom.
My belief is that the dip in safety factors was borne out of an arrogance of engineering. There was an established thinking in the post-war world that anything could be accomplished better, cheaper, and faster with 'new' technology. We had defeated armies, harnessed the power of the atom, and built a fleet of ships and planes faster than had ever been done before. We were bold, we were daring. We were ignorant. The technology that won the war was built to outlast the Germans, not to outlast the generations. Centuries of engineering expertise that had taught us to overbuild, overbuild, overbuild was tossed on the rubbish pile. We could build better bridges with half the materials, we had the technology, or so we thought.
To be fair - they have held. They simply are designed as disposable architecture. Designers at the turn of the century designed their bridges to be monumental - to last for generations. Designers of the post-war era designed for the here-and-now. Any engineer worth his salt knows that what goes up must come down, and that a structure is built as a time delay to control the time frame until the eventual return to Earth. the engineers of the turn-of-the-century and the engineers of the post-war era merely disagreed as to what a reasonable timeframe should be.