Great question! There's really no simple answer to this unfortunately also I don't think I'm the best person to answer this question, but I'll try my best!
Water is a basic human right. In most countries, water is a public good that the government subsidizes and, to some extent, the market is adjusted to allow vulnerable households to afford the resource as well. If we have a 100% free market-based solution then in many locations the poor and underprivileged have a considerably less access to this vital resource (some won't get any). An example of this can be seen in India. Many districts there suffer from intermittent water supply (where the local piped water comes at irregular intervals instead of continuously). These people might not be able to shower regularly and they aren't given a schedule of when the water will come, so they can't schedule their day or week. Many of these people might even have to leave work for a day (losing that income and economic productivity) and jump out to fill up their water tanks in their homes. As they don't know when the water comes (no regular scheduling), this uncertainty can be nerve racking and (even if you're trying to stretch it out to last) may not be enough to last your family until the next "supply". In these cases, you call in a water tanker to refill your tank but even then you don't know if it'll come.
On the tanker truck company's side, they don't want irregular domestic clients, they want more regular commercial clients (because the total customer value is much higher, you get more money from those clients than a household that needs a one-offer). So those companies aren't really incentivized to service residential clients. So if another call comes in, those commercial clients would take higher priority than those vulnerable households.
Those solutions don't really help this problem, and this problem is just going to grow for the foreseeable future. Not exactly what I'm talking about, but IWA recently published an article about how many cities are shifting from 24/7 to intermittent water supply solutions [0], and if this becomes irregular and the system starts to fail, then this will just keep that problem growing. One of the projects I'm involved in works on trying to solve this exact problem and we recently got some support from the US National Science Foundation as well as the Gates Foundation. However, there's still a long way to go. (Sidebar: Anyone want to give us more funding or help? Send me a message!)
[0]: https://iwa-network.org/running-out-of-water-cities-shifting...