Lack of spares for Datsun Go

Lack of parts for my wife's vehicle. The check engine light came on and when we took it in to our local Nissan in the Westrand they advised they could not obtain the lower oxygen sensor replacement part and that it would take 2 - 8 weeks on backorder. Whay does Datsun SA / Nissan SA not keep spare parts for this vehicle? They are happy to sell them but you have to wait an eternity for spares. Last year we had a small accident with the vehicle and it took over 4 weeks for them to supply two new wheels!