Why would BT not be able to find a spare pair?
Three possible reasons -
1. When the GPO / BT were building the local telephone infrastructure, the internet was not even thought of. Nobody had more than one phone line in their house. These days it is not uncommon for people to have two or more phone lines in their house. Also business use of telecommunications has increased hugely over the last 20 years. So sometimes when someone requests an additional line there are simply not enough pairs in the ground.
2. Copper pairs are subject to corrosion and can occasionally fail. In this case BT are obliged to repair the fault quickly and for the reasons above may be unable to find an unused pair.
3. Sometimes, even when spare pairs exist all the way back to the exchange, there may be a lot of work involved in connecting up the pair. The engineer may have to visit several cable junctions, some of which may be at the top of poles, some of which may be in green BT cabinets, some may be underground and some may involve stopping traffic etc. to lift manholes. In such a case it is often easier for a lazy engineer to DACS an existing pair.