There can be one more theory that :

At initial level the chances of injuries are low because the players are playing at low level a less competitive and physically demanding football...But as you move up the level the chances of injuries increase so in order to keep the chance of injury same as earlier upgrade your
TURF...

Other wise at higher level you will have more injuries than at lower level...So TURF upgrade only stop the increase in injuries that can happen at higher level......