The major issue was the Susie safety nuts who felt that without telling people how fast was reasonable that it would confuse people, the court agreed.
When the government gets to tell people what is reasonable, you have lots of problems. You see, in this country, we have a Bill of Rights that provides for unspecified rights "retained by the people" (9th Amendment), and "reserved to the people" (10th Amendment). The highest law in the land says the people get to tell the government what is reasonable, not the other way around.
The "speed limit" should be instead taken as a "recommended speed", which should be based primarily on factors such as the engineering of the road, and the likelihood of animals or people being encountered. There will be circumstances when it is appropriate to drive faster than the recommended speed, and there are circumstances where it is appropriate to drive slower.
Let the police show that somebody is driving very differently than other people do on that road, and doesn't have a good reason for doing so (in the eyes of a typical person, informed of the facts, and with good judgement), then the government can take punitive action.
This approach is consistent with people driving at reasonable speeds, as defined by the people and not the government.
But the last thing the US legal profession wants is for people to realize the 9th Amendment exists, since that is the gateway into some very scary basic rights (for them), such as the right to ethical practice of law
Far simpler (for the legal profession) to just move back to the status quo, where people -- in practice -- drive as if the speed limits were merely recommended speeds, and hope the police are being equally reasonable*. It's a really bad policy, and hugely unethical for the legal profession. By effectively forcing people to routinely ignore one part of the written law, they make many people scared of the legal system. They also send the message that people can not engage in reasonable conduct: the legal system can and will punish them even when they are being reasonable. These two factors create an artificial demand over the long term for the services of legal professionals to "protect" people from their own legal system. Welcome to the Land of the Lawsuit.
In short, the court was in a position of ethical conflict of interest with respect to this ruling, and did the wrong thing (something that happens an awful lot in US law: except in extreme cases, the legal profession looks out for its own).
*Defunding the police helps here and seems to be popular, but causes all kinds of other problems. Not a good solution. There's also the ethics issues associated with government trying to keep the money it gets from fines, which increases the amount of ticketing going on, is also a 9th Amendment violation, and creates contempt for the police that makes their jobs a lot harder, but that ethics problem is a subject for another day.