There are 150,000 public water systems in the US, these include things like schools on well water, shopping malls, etc. They are regulated by the EPA the same as any other system, but they do not serve a large and consistent amount of people. If we remove all of the non-continuous use systems, we arrive at a staggering 50,000 community water systems. The top 8% of these water utilities serve 82% of the population, meaning that the big systems are very big (and generally well funded) and the smaller systems are vastly more numerous and much smaller.
While having a lot of physically separate systems for water treatment might make things easier, this is not a McDonald's franchise (only 13k of these btw). Unlike McDonalds, water utilities can be publicly owned, privately owned, publicly owned and privately operated, special-district owned, cooperatively owned, or be a non-profit corporation. Each of these districts is governed by the EPA, there are laws on the books where action must be taken if a contaminate is tested over limits. While the EPA requires these utilities to test their water, when a test is failed - what is next?
With a system so fragmented and filled with people, we should expect a certain amount of friction, but when you combine this standard enterprise friction with the need for political action (read: funding), things quickly slow down.
Compression
When these market and political forces intersect, we arrive at a dangerous place. Water systems are built for equilibrium, they work best when there is a constant supply and demand, and when there is consistency in the water supply. If you change any of these variables, massive issues can arise. While the infrastructure of water delivery is invisible, that does not for one second mean that it's not expensive.
A vicious cycle can emerge if, for instance a city loses population. Detroit lost around half of its population in 30 years. The water utility was now receiving only about half of the demand that it was built for. This causes the per-customer pricing of any type of system maintenance or upgrades to skyrocket. Consumers don't want to pay more taxes to fix something they can't see, and politicians can't sell themselves on something people can't see. This situation of population decline and under-invested water systems is prevalent in the US, especially in the rust belt and deep south.
It is also important to remember this happened in a city with direct access to the largest freshwater bodies on earth.
No One Goes to Jail (or loses their job)
When a major water system failure happens, even when there is evidence of fraud or misconduct, the people in charge rarely experience the consequences of their cost to public health. The structure of the EPAs enforcement of the law is not heavy on prosecution, after all we do want the problem solved rather than someone in jail. However in an environment of degrading infrastructure, emerging hard-to-remove contaminates, and ever tighter budgets, I don't know what we should expect from water utilizes. They seem to be backed into a corner, and even with massive federal grants for modernization, it is only a band-aid on a system that even when maintained properly is unprepared to remove modern contaminates like PFAS and other endocrine disrupting chemicals.