The reasons are:
1. sodium is the major cation
2. Mg and Ca is added to a comparable physiological concentration. Usually the Mg level is slightly lower than plasma (around 0.5 mmol/L) as PD paitents tend to have hypermagneseamia. There are variable Ca concentration from 1 to 1.75 mmol/L for correction of hypocalcemia or hypercalcemia. The lower concentration solutions is getting more popular as a mean to minimize vascular calcification in dialysis patients.
3. Chloride is the major anion
4. lactate is used as the second major anion. It was used instead of HCO3- as Ca will deposit with HCO3-, and after absorption, lactate will be converted to bicarbonate in the liver. Unfortunately, lactate makes the PDF slightly acidic and itself has also been demonstrated to be bio-incompatible to the mesothelial cells. So, newer generation biocompatible PDF containing biocarbonate has been recently developed.