US Bishops on Stem Cell Research

The US Conference of Catholic Bishops (USCCB) recently made a statement regarding stem cell research. It is worth reading at length. Stem cell research has caused quite a bit of controversy, and rightfully so. It seems that the stem cells from adults as well as umbilical cord blood has proven beneficial while the stem cells from embroys haven't been proven to be effective. Why would the scientific community insist on "harvesting" stem cells by destroying the lives of human embryos when umbilical cord blood is easy to come by?