Doing five minutes research it seems that it's just down to the Piccadilly line being busier than the Bakerloo line - given that the Bakerloo being duplicated i

Doing five minutes research it seems that it's just down to the Piccadilly line being busier than the Bakerloo line - given that the Bakerloo being duplicated in parts by the London Overground. The Central line are on much more modern stock - the 1992 stock, and are currently undergoing a renovation programme, so the need to replace them is less pressing.