Here's why:
* Shaping involves reinforcing successive approximations of a desired behavior. This means you start by rewarding any behavior that remotely resembles the target behavior, and then gradually increase the criteria for reinforcement as the individual gets closer to the desired behavior.
* Successive approximation describes the process of getting closer and closer to the final target behavior through these incremental steps.
Essentially, they are two different ways of describing the same process of training a behavior.