Why does well water disqualify the phosphate control system?

I am entertaining a salt system as we will be away from the house a lot this summer. Looking for something that is easier to maintain as we are away. I have been doing the chlorine method for quite some time and its is easy when we are here.

We are 36,000 gallons concrete pool.

Adding a robot to help clean, the Polaris 380 died along with the booster pump. Going with a 2018 Dolphin Premier Robotic In-Ground Pool Cleaner.

Mike