Without publicity the donations dry up. At the moment the total is ~$700,000.00—well shy of a million. If Trump drops the matter, the widow and Wilson will have to massively up their publicity efforts/Trump attacks, in order to keep the $ coming in. They might even be forced to invent new lies. There’s not much that some people won’t do for a payoff of several hundred thousand dollars.
And the publicity would have gone away had he dropped the need to get even.