It could be argued that the explanation isn't sufficient but it is in the film, in the conversation between the Ranger and Son No. 1:
-Well, what are you gonna do?
-Not a goddamn thing. D.A. says there ain't no crime here. Every one of them gals was swimming in alcohol and floatin' on weed, and old Hooper in there came out clean as a whistle.
-Now, you actually think that he premeditatively murdered them gals?
-Well, I can't prove it.
Yeah, I think the only thing we can dream of getting that bastard on
is vehicular manslaughter for the hitchhiker in the death box. That was just plain old, goddamn reckless endangerment. But I got me a goddamn bartender
gonna testify that ol' Stuntman Mike didn't drink a drop all night. And his passenger was left stranded by her date, in the goddamn rain, no less,
and she asked him for the fucking ride. Now, on paper, this is gonna look
like he was just trying to help her out. I mean, that's the way the jury's gonna see it.
-So, what are you gonna do, Pop?
-Well, I could take it upon myself to work the case, Search for evidence,
you know, prove my theory. Alert authorities. Dog that rotten son of a bitch. Wherever he goes, I go. Or I could spend the same goddamn amount of time and energy following the NASCAR circuit.
Seeing that Mike was sober and the girls weren't, and this happened at night on a deserted road and all other witnesses are dead, the police suspects Mike but concludes that there's no case.