Solid vitamin C is relatively stable, but it decomposes rather quickly when dissolved in water. Factors such as pH, temperature, oxygen, and the presence of catalysts (iron, copper) influence the decomposition process. The lowest rate of oxidation is observed at pH 3, where vitamin C solutions are the most stable. Raising the pH to 5 increases the oxidation rate by a factor of 2.

https://pmc.ncbi.nlm.nih.gov/articles/PMC3510389/

The study uses particularly clean water (clean enough to be suitable for medical injections) with a pH of ~7.4. At that acidity and a temperature of 20°C (≈70°F), it takes about 95 days for the vitamin C to decay to 10% of its original concentration, or 28 days to reach 50%.

Normal drinking water has a pH of 6.5-8.5, but also contains a lot of other substances, which might increase the rate of oxidation. Given the potential time between treatment and consumption as well as the fact that people might boil it and increase the rate of decay that way, it’s just not as economical to add ascorbic acid to the water supply if only a small percentage of it will ever reach the consumers.

Additionally, the exact dosage will be hard to control, leading to a risk of excessive side effects such as kidney stones. People with a specific enzyme deficiency may also suffer anemia as excessive doses.

Compare that to, say, lemons, whose juice has a pH of ~ 2.4 and renders the vitamin a lot more stable. If you want people to get a good intake of vitamin C, tell them to eat fruits and vegetables, preferably uncooked. The vitamin C dosage you’ll get from that will hardly lead to megadoses, unless you eat such vast amounts that you’d probably get other problems anyway.

The reason fluoride is added is that it’s quite stable, safe and effective, while also being fairly cheap.