Highways were constructed in regions with sparse populations or in urban areas with little political power (primarily black and Latino neighborhoods). Basically, areas where democracy didn't have to function because there was no democratic power to block it. Whereas nowadays, with higher levels of democracy (unequivocally good) and local control (more of a mixed bag), massive infrastructure projects are harder to accomplish. Plus, the 50s had the benefit of a booming postwar economy and the national cohesion (at least among enfranchised Americans).