They're claiming that construction costs raised substantially and unpredictably and due to that can no longer fulfill their obligations.

I caught two important points on that argument though.

1. The original funding was granted through auction, which incentivised ISPs to underbid what costs would have been even under the best circumstances.

2. A separate coalition of ISPs who did not win claim that these market increases were not as unpredictable as claimed, and in fact were factored in by the more responsible participants in the auction:

The Coalition of RDOF Winners said these cost increases "could never have been anticipated by the Commission and RDOF winners prior to the auction." But the WTA said that isn't true and that its own telco members "and other responsible bidders factored these likely future cost increases into their Auction 904 bid strategies, and stopped bidding when the bid prices became so unreasonably low that projects were no longer financially feasible or sustainable."

I'm inclined to believe the WTA. The auction occurred in October 2020, well into COVID when its volatile impact on supply chains and the like was apparent, which I was thinking might have been the crux of the winners' argument.

No sympathy here for the winners, they made a deliberately reckless gamble and these are the consequences. But also it was a dumb way to grant this funding to begin with.