Because the buyer attached some expensive conditions to the offer. (Put on a new roof, the old one is at end-of-life). It is not really a full-price offer when that happens, and the seller has an option to refuse the deal. Or make a counterproposal.
You suggested that the seller realized they underpriced it?
Maybe the seller did underprice the house. Too bad for them. But even very experienced real-estate professionals can make large mistakes estimating the value of a property. It is almost always better to close the deal and move on.
If the buyer adds those conditions, then it seems like it doesn't trigger that clause in the contract anyway.
I'm not talking about a mistake, I'm talking about listing low to get a quick sale.