Apart from the legendary beasts and a few other divinities, Abra is the only pokemon which has truly learned the fear of man. While most pokemon attack trainers, and a few (like Chansey and Tauros) wait to determine if the trainer is hostile, Wild Abra respond to any encounter with a human by teleporting away.
For generations, it was believed that this was because Abra can only use this technique, but recent observations from far-off have revealed that they are also capable of using each and every technique known by a Kadabra of equivalent experience. It is simply that Abra never use them in the wild, and once trained refuse to use other attacks until a trainer has proven his or herself by sticking with it until it evolves.
Abra are the youngest form of the super-intelligent Alakazam and possess a remarkable talent for mind-reading. Perhaps they fear human contact so much more than other pokemon because they alone can truly see the wretched minds of mankind. Then again, perhaps this is only a childish, black-and-white understanding of human nature: Abra, after all, evolve into Kadabra.