Most of the stem cells that work are adult stem cells. The fetal ones never work very well. My theory is the fetal ones are not mature. The adult ones are that is why they work better.
Yes I know it is silly to say that cells "want" something but that is what they persistently try to become. Teeth, hair, bones, skin. They keep trying even when they have been torn apart.
Adult cells have settled down. They have fulfilled their biologic imperative and now if given another job they will do it.