Several reasons:
- The Polmont crash of 1984, in which a DBSO derailed after hitting a cow, put paid to coaches being used as driving vehicles (DVTs don't count as they are better designed for object strikes, and are ballasted)
- Redundancy. Having two locomotives provides a back up when one locomotive fails (see HST)
- Traction; having the power spread over more axles improves traction and performance; more relevant to MUs though (see 390s, 220s, 221s, 222s v HSTs, cl 91+mk4s).
Distributed traction is becoming the preferred setup for modern stock, due to improvements in reliable traction technology. The spread of the traction equipment along the train reduces axle weight compared to having a heavy loco at one end, which means lower wear to the rails, which means lower track access costs to the operator.
For freight, it's all about cost - two medium-power locomotives cost more to run in a freight train than one super powerful loco; and there's the issue of controlling the rear loco, without wiring every wagon with multiplex capability. If one loco (or sometimes two) can do the business from the front, there's no need for a rear loco.
Edit: except the steepest gradients - I believe DB Schenker have a small number of 66s which deputise as banking locos that latch onto the rear for steep hills, which have an automatic un-coupling capability.