Usually ABS only activates when a sudden brake force is applied. If you were already on the the brakes and increased pedal pressure, the ABS would not have activated. When you were trying to test it later, were you just stomping the pedal, or steadily increasing brake pressure? If my theory is correct, if you had released the pedal then re-applied the brakes, the ABS might have activated.
Either way..it really sucks that you got into an accident.