It's easy to say this in hindsight, but how many pools/hot tubs have employees monitoring an alert panel around the clock for water quality metrics of the water?

Countless systems are relying on email as a means to alert people to issues, for better or worse. Having a physical alarm for everything is rarely practical for a myriad of reasons.

I worked in a data center 20 years ago that had alarms on the wall for when the halon was preparing to be released for fire suppression. There are likely dozens of things that could be monitored, but it was only the catastrophic thing that once it started, there was no stopping it, that got lights on the wall.

Too many loud alarms make people start ignoring alarms. I would guess that there is a fairly large margin of error in pools before there is a health risk, so it likely wasn't deemed alarm worthy. As someone who's been in a pool before, I'd also imagine that people would notice and say something, or it would be unpleasant enough to not want to spend time in it. That's what I find most surprising... that people weren't immediately turned off just being around it, to the point of staying out.