Why hard water is the suitable feed to be deionized via deionizers. And why soft water is not?

It takes a while to learn what the words mean, and it is almost impossible to get straight information about water filtration in general. Some dealers will not even tell you if the filter you are looking at will fit into the canister you are looking at.

"Hard water" means ions of calcium and magnesium. "Deionize" means to remove all ions of all types. "Soft water" means reduced magnesium and calcium, but a water softener does that by replacing them with sodium ions (salt). Salt kills plants and causes heart attacks in animals. So you would not use artificially softened water, but naturally soft water is good.

To repeat: artificially softened water causes heart attacks. Don't use it for watering plants or animals. Only for washing.