Here's a few thoughts: -
1. Capital outlay control in uncertain economic times.
2. DDA deadline, i.e this years order heavily weighted to D/decks, which cost more than S/decks(This is a 'one-off').
3. Average age and make up of Existing fleet, it is now fairly modern and increasingly standardised, i.e oldest Darts are '53 reg.
4. Older vehicles are lasting longer, there seems to be no urgency to replace '51, '02, '52, '03 Solos for instance.
5. Future Franchising issues, if for example Greater Manchester put all routes in GM out to tender, S/C might want to lease vehicles as per London.