(not a lawyer, opinions are my own, yada yada) The executive branch has broad power to choose which laws to enforce and when and how to enforce them within certain limitations, such as not being allowed to racially profile. It's worth asking whether the pros outweigh the cons for the status quo, but that system does have some benefits. Here are a few:
(1) It offers a check on legislative and judicial power. Consider ~2005-2018 where some states had legalized marijuana but the federal government explicitly had the power to enforce marijuana laws. Selective enforcement allows those agencies to decide to uphold states' choices.
(2) To the extent that human cognition is a necessary component of the legal system, it's cheaper and easier for everyone involved to shift some of those decisions earlier in the process, much like shifting software bugs earlier in the development process. E.g., there was a period of time midway through the 2020 lockdowns in CA where the DMV might have been trying to process your new tags for half a year, where you might have paid for those tags long before they expired, and where you were technically breaking a law by not having new tags available to display on your car just after the extended governor's order halting enforcement expired. You'll get off scot-free if you take that in front of a judge, but it's cheaper for everyone involved for the police to just not write a stupid ticket in the first place.
(3) As a practical matter, not all laws can be enforced, so some level of discretion is necessary. If every CA police officer were working 24/7 then every hour of every day they'd on average have to police 320 car-miles driven on CA roads. Moreover, every ticket they write is a similar burden inflicted on the rest of the system.
So what happened in the example traffic stop? It could be a lot of things, but some combination of (1) and (3) seems likely. The officer might have (1) determined that the >=80mph punishment in that state was excessive given the circumstances and limited the legislature's ability to use that power here and (3) decided that if the speed were slightly dropped there would be a lot less incentive to fight the ticket and use up other government resources.
It's probably also worth noting that having a person on radar still doesn't necessarily suffice. In many states a +/-5mph tolerance is allowed in a speedometer, but speeding is still illegal. I'm not sure if the intersection of those two concepts is well tested in court (e.g., does intent matter in this case?), but in practice many cops, prosecutors, and judges are more than willing to drop 5-10mph off a ticket to accommodate such things.