Law enforcement could get a search warrant from a judge, but would prefer to have consent. There are also difficulties inherent in trying to go back to the judge and seek a second search warrant to search the same place, for the same thing. Finally, law enforcement has a dilemma -- the could be nothing there, less than a handful of sets remains, or 50+ sets of remains. If it is the latter, this would probably be the largest crime scene investigation in State history. Given that the alleged perp is dead, better to line up resources ahead of time.
Of course, it is better to get landowner’s permission. I’m a landowner and don’t even like it when surveyors do their stuff without informing us. But they began investigating in 2021! Getting the pooches to sniff it out just happened lately and got the ball rolling.