Because of ethical considerations regarding whether or not fertilizing eggs for the express purpose of killing them to harvest stem cells represents infanticide. It goes back to the age old argument over where life begins, at conception, birth, or somewhere in between. It’s a pretty darned hard question to answer, so many ethicist advocate exploring alternatives instead.
Frankly, I have no problem with embryonic stem cell research. However, in defense of the decision to restrict that line of research, I can’t help but wonder if without those restrictions the phenomenal research into other sources of stem cells (such as our skin) might not have taken place because all those talented researchers would be focusing on embryos instead.