Summary: An experiment that looked for WIMPs (weakly interacting massive particles) found none with masses above 9 GeV.
So either they exist but are too rare for the experiment to see, or they are massive but not that massive*, or dark matter isn't WIMPs (or dark matter doesn't actually exist at all).
Seemingly, the most mundane possibility here is they exist, they're not rare at all, but "not rare" on the global scale of all galactic and larger superstructures in the observable universe doesn't mean you're necessarily going to find them in South Dakota in some specific 280 day window.
So either they exist but are too rare for the experiment to see, or they are massive but not that massive*, or dark matter isn't WIMPs (or dark matter doesn't actually exist at all).