I would think a large part of it is simply the universities want to offer tenure to the most competent researchers. And it's harder to judge whether a researcher who mostly gets null results is doing a good job.

Perhaps this means it really does have to start with journal publications though. If journals value null results, peer reviewers will sharpen their ability to distinguish null but well-run experiments from ones that failed simply due to poor execution. Then employers can use published null results as a positive signal that a researcher is indeed doing good quality work.