Negative results tie directly back to DARPA’s mission to prevent technological surprise. If DARPA finds that a technology doesn’t work, the chances that someone else will be able to get it to work are also much lower. They can effectively check the box of “we don’t need to worry about that until something changes.”
The ability to not worry about something is why they will fund truly crazy things like Reactionless Drives even though they expect them not to work.
The question is whether there is a way to replicate this incentive structure