Probably for the same reason that people didn’t challenge town ordinances that required firearms to be checked in with the Sheriff back in the old west. I dunno. Maybe people were too poor or too ignorant of the legal proceedings to challenge such things. Or maybe it’s something else. it’s a good question, though.
pkbites has the right idea, though I would say it was more a matter of culture than being poor and ignorant of legal proceedings. I didn’t grow up in Montana, but I was raised — and taught to drive — by a Montanan (my father, on whom be peace) and spent ten years there from the mid-60s to mid-70s, so I did acclimate to the culture. And the Basic Rule was just something understood and accepted.
Another possible factor was that before Ted Turner et.al. “discovered” Montana, it was a backwater that didn’t show up on anyone’s radar so the existence of the Basic Rule wasn’t well-known. But its reinstatement at the demise of the double-nickel became somehow newsworthy, and chaos ensued.
ETA pure speculation: the Supreme Court may have been influenced by the concern that if they had gone the other way, it would have reinforced the idea that “Montana has no speed limit! Yahoo!” And all Helena would indeed have broken loose.