The primary reason would be the ethics, since getting embryonic stem cells requires destruction of a young human embryo.

The biggest misconception people have about stem cell research is that it is only embryonic that are useful.  In fact, other stem cell types are proving to be much more useful.  The best stem cells for patients are Adult Stem Cells; these are taken from the body (e.g., bone marrow, muscle, even fat tissue) or umbilical cord blood and can be used to treat dozens of diseases and conditions.  Over 1 ½ million people have already been treated with adult stem cells. (versus no proven success with embryonic stem cells.)  Yet most people don’t know about adult stem cells and their practical success.