Underwriters, many times, are businesses that, like the commercial radio counterpart, has seen radio spending decline substantially. Even underwriters that may be non-profit have likely seen substantial drops in their contributions from contributors. Certainly listener donations are down as well. Much of public radio's dollars come through grants and donations from non-profits and companies that may not be classified as traditional businesses. Underwriters are largely businesses reliant on customer count and consumer spending.
Many stations on the commercial side have seen easily a 50% loss in advertising revenue due to the pandemic. Of the LPFM stations I know, it's about a 75% loss. I only know a couple of public stations. One shared with me they dropped from $160,500 to a projected $40,000 assuming things do not improve in coming months.
Radio, both public and commercial has struggled to win back dollars since the housing bubble burst. The pandemic is just salt in the wounds.