Just a guess, but I think most people that are injured during aircraft evacuations are elderly people, My elderly dad fell off a step ladder some years ago and got banged up. Having to take a 20 to 40 foot slide ride at that age is an issue..
We were told that most people were hurt when they landed on people who had not gotten off the bottom of the slide, causing a pileup at the bottom. This is why our first instructions after "Stand back! Stand back!" while opening the door was "You and you, stay at the bottom, pull people off and send them away!" followed by "Jump! Jump!" Elderly individuals would definitely have difficulty getting up off the bottom of the slide.
Nowadays, I wouldn't be surprised if people got hurt because passengers are trying to take their 50 pound hard side rollaboard down the slide.