As the paper outlining the NVSS notes, 1.4 GHz happens to lie in a sweet spot of radio frequencies. It falls into the "intermediate-frequency gap" in which many of the sources of interest display little variability, making the survey useful for future astronomy. At higher frequencies, sources tend to exhibit intrinsic variability; at lower frequencies, effects from interstellar scintillation become more drastic - something true of many processes involving the interstellar medium.
The reason for the precise choice of frequency is the abundance of L band radio receivers that cover it. It corresponds to the famous 21 cm line, the signature of neutral hydrogen (e.g. in the form of clouds and HI regions). Since Galactic and extragalactic neutral hydrogen are commonly studied - and since you can use the same frequencies to observe many broadband sources, like pulsars and fast radio bursts - L band receivers are ubiquitous. Hence, at least some of the VLA's observing capabilities in this intermediate frequency gap were centered near 1.4 GHz, making it a reasonable choice.