Because the NFL is more concerned with lawsuits than player's longevity. If the lawsuits start flying around knee injuries as well, then they will take action.
In all fairness, they have been cracking down more on the shots to the knees, but not near enough in my book. Not just talking about the crack back blocks, but the DB's targeting RB's knees even when they are in very vulnerable positions. They know what they are doing, but it is legal, so they are taught by some to go ahead and do it. GB CB's used to always target AD's knees.
Expect more of this with all of the high hit and helmet rules. Defenders will hit low; blocks will go low. Knees will be destroyed; careers will be ruined.