Why do foetuses have different haemoglobin to adults?

Normal adult haemoglobin is primarily comprised of 2 alpha and 2 beta subunits and foetal haemoglobin has 2 gamma subunits instead of the 2 betas. Now what this does is increase the binding affinity of this haemoglobin to oxygen. The reason this is necessary is due to the principle that the foetus has to obtain everything it needs to grow and thrive from the mother's blood, via the placenta, though the blood never mixes. The foetal blood is a relatively low-oxygen environement, which means in order for it to be energetically favourable for oxygen to move from the mother's to the foetus' blood, the foetal haemoglobin must have a significantly higher affinity. This property can be exploited in the treatment of sickle cell anaemia.