Why does fetal haemoglobin have a higher affinity for oxygen than an adult haemoglobin?

In order to survive. By the time the blood reaches the placenta there is a lower concentration of oxygen in the blood, the fetal haemoglobin has a higher affinity for oxygen so that the hb can bind to oxygen at the lower partial pressures of oxygen in the mothers blood. Hence to facilitate the transfer of oxygen from the mothers blood and to the developing fetus.