It is not just one thing.
1. Money. These guys get paid so much more than their forefathers it is ridiculous. Back in the 40's-70's they had to play so that they could get paid. And it was every down. Now they have all these situational substitutions many don't play much more than half the snaps.
2. Technology. Since they can diagnose the injuries better today than in antiquity they have a better understanding of what the injury is. In the 40's they would have said sprained knee instead of a torn ACL.
3. Legal system. With the lawsuits hanging over the NFL they want to make sure they come across as the Caring Corp.
4. Attitude. Kind of ties in with #1. If players have a good thing going they want to milk it for as long as they can. Sitting out the remainder of a year can lead to a longer career. Whereas playing with an injury could cause more harm and shorten a career. What would someone like Gale Sayers have been able to do with he had today's technology and not tried to play though some of his injuries?
5. Conditioning. Like Pack93 says the shorter training camp and shorter practice week may well have an impact on injuries.
6. Physical abilities. The players are bigger, stronger, faster today with better equipment. There is more opportunity for injury. I knew a guy who played for the 1970 Baltimore Colts. (I saw his super bowl ring.) Later he played a couple of years for the New England Patriots. He was a HB. I could not believe how small he was compared to the players in the 90's.
7. Dirt. I like the approach Beast has. Without natural fields they don't have dirt on the sidelines like they use to. Coupled with that is the plastic fields are not the same as the mud fields. But we all saw how upset NFL Inc was when GB hosted the NFC CG in the 90's with a muddy field. There is nothing wrong with a field that is more mud than grass in Dec and Jan. Other than the fact that it doesn't look pretty on tv. That and the players won't run as fast so there won't be as many injuries.