First we had volunteer nurses with no degrees - just with a heart to help the sick and dying. Second, we had diploma nurses that were hospital trained nurses - eventually doing curriculum and bedside care. Third, came the Associate Degree Nurses who have a combination of college courses and clinicals. Then, fourth, we had the upsurge of making a four year program for nurses that the AJN wanted to become the elite group, where they hoped to do away with the diploma and associate grads, making the BSN the basic entry level into nursing today.
My opinion only now is this: With changing times and more and more women wanting recognition in their own chosen career fields, the desire to always "one up one program over another" was to signify one being better than the other, therefore more deserving of higher recognition and pay.
The reason the three programs still exist? There's obviously a need for the three programs due to affordability among those desiring a career as a nurse. The vast age ranges of students entering the nursing profession dictate a need for all three levels -STILL. The AJN BSN promoters would like to do away with anything shy of a four year program for nurses, but the supply and demand have yet to allow this transition to take place.