I wonder if part of this has to do with the more complex, piecemeal nature of medical debt and these buybacks.

How I understand several of these groups (I'm not certain of the one in the article) do these debt purchases is that they purchase debt that has already been bundled together and resold. So when a person owes medical debt to a hospital, a doctor, a surgeon's practice, a pharmacy, etc, they may owe 15k in total, and these buybacks only cover one resold debt, they may have their hospital debt forgiven, for example, but still own 10k in total to the 3 or 4 other creditors.

Contrast that to a mortgage debt, for example, where paying off the mortgage relieves all anxiety around that debt, and I can see how the difference in complex, multi-creditor debt versus simpler, singular-creditor debt could help address the findings of this study.