I seem to recall that part of the DoE's selection criteria is to ensure a competitive market for future contracts (since they will always be buying more supercomputers). Given the recent dominance of Nvidia in the market (see https://www.top500.org/lists/2019/06/), it probably made sense for them to ensure some contracts went to other suppliers (AMD for Frontier, Intel for Aurora) to ensure that Nvidia doesn't establish a monopoly on future tech.

Additionally, these sorts of supercomputers are also a way for governments to implicitly subsidise their tech industries: when viewed through that lens, spreading these contracts around makes a lot more sense.

Word on the street is that Nvidia insisted that they pay the full price for the GPUs this time and were giving them at a discounted rate prior.