What a very very interesting question slinky and the reasons are multifactorial but i will give you what i consider to be the most important in my opinion.
The young doctor of 40 years ago worked for a consultant and it was his/her job to know and look after the consultants patients. Know their name, age medical history, medications, prognosis and future plans. This would take half an hour more more going through one patient's notes, but at the end of it you would know everythng about that patient which was essential, as you would be responsible for that patient 24/7.
No days off, no shifts, but the occasional weekend off if cover was available.
Now the hospital doctors are by law only allowed to work so many hours a week, weekends off, study leave, day off to go to grandma's funeral, day off to get married, counselling sessions for stress due to overwork........etc......so you see, the patient will be seen and interrogated by any doctor that happens to be on and he starts the history all over again.