I think the main difference is, back in the day, if you hurt your arm you just stopped playing or pitching instead of going to a doctor to get surgery. It was more a case of genetics than care.
I think reporting injuries along with having the surgical/rehab option have a lot to do with today's increased numbers.
Growing up, I never heard of one kid in another town who hurt his arm but I knew a few who stopped playing/pitching because of it in my town.