why does continuously fertilising the soil with superphosphate make the soil acidic?

Answer 1

The same principle that makes acid rain is involved. The sulfates in the fertilizer react with water in the soil to produce sulfuric acid, H2SO4.

Answer 2

Most of the chemicals we pour into the soil are artificial. We need to get back to a more natural ways to produce food. Several farms which went back are now out of business by government orders. The land does not react well to prolong and intense use of chemicals. But you should asks others their opinions before seeing the real answer.