The cells came from the mouse used, and were accepted more easily.
Embryonic stem cells are often said to be useful, because they don't have an identity yet, but the immune system can be hard to fool, and as every person has their own supply of stem cells, at different stages of advancement, . it makes sense to use cells that will be recognized. The field is only in it's early stages. While Embryonic stem cells seem to have had potential, there are no doubt cells that are just as pliable within each of us, and using them for any treatments, will mean a number of other problems wont have to be overcome.