Probably because the NTfL studies forecasted much larger opportunities on Piccadilly (60% capacity increase versus 25 on central and bakerloo). So they prioriti

Probably because the NTfL studies forecasted much larger opportunities on Piccadilly (60% capacity increase versus 25 on central and bakerloo). So they prioritised deploying the new trains on Piccadilly. All three lines, and Waterloo & City, are planned for upgrade.