Earlier quoted context omitted.
Because water is so cheap (cheaper than dirt) that it’s not punitive at all to pay for twice or ten or a hundred times normal.
On the one hand, if water is so cheap, why do you need to punish people for using a bit more of it, then? On the other hand, if water is so scarce that you need to discourage people from using a bit extra, why do you price it so cheap in the first place?
Everyone needs water to live. The actual price of water, fairly pricing in long term externalities, may be too much for a family of four to afford their 8 or 10 kgals a month. At least for many such families.
But when you price it at the marginal cost to pump out the extra gallon, it becomes economical to do silly stuff like grow water intensive crops in the desert and draw the city's aquifer down.
Hence the conundrum. Price it fair, some people can't afford flush the toilet. Subsidize it at a flat(ish) rate, or even just ignore externalities, some people can afford to destroy the commons. So, conservation pricing.