I think two factors contribute:
1. Teams abuse the DL. Players with a nagging injury are placed on the DL to make roster room. I don't really know, but it seems to me that the number of options on young players must be fewer than in the old days. Therefore, a lot of players who might have been sent down before are just put on the DL for fourteen days. You even see DL stints that can be partially attributed to a need to maneuver in a fifth starter during a tough stretch in the schedule.
2. I think the extra weight-lifting and strength training contributes. While good training undoubtedly avoids many injuries, I think today's players are so bulked up that it puts strains on places that never acted up on the old players. Example: Bo Jackson. It didn't contribute to the injury-ending hip injury, but he was so bulked up that he was constantly pulling a hamstring when he was playing.
Dave N