I guess because percentage of the USA population believes it is killing a human, even though unborn. They believe even a zygote, virtually a clump of cells, deserves human rights and a right to life. This is a religious and philosophical question. This shows you how secular the USA claims it is, it is highly religious.
When Roe v Wade was in effect, the legal test was--abortion was banned after the third trimester of a pregnancy, this basically meant that after the third trimester the baby could survive outside of the womb (viability) with the help of modern science, the baby would just be extremely premature. Therefore, at that stage, the baby would be considered a human and hence given the human fundamental right to life.
But for a real answer on this topic, you should consult a religious leader, a doctor or a sceintist. A lawyer has no real business in understanding the real answer to this question.