Ebola's strengths are also its weaknesses. It kills fairly fast, allowing only a short period for transmission. It weakens the victim so fast that they don't move around much once symptoms appear. In Africa you'd see entire villages vanish but neighboring villages remain untouched. In the US, our constant travel and close contact make it a more serious problem, though.
I'm not sure the US has much interest in biological weapons. If extermination is our goal, we've got plenty of ways to do it in a far more controlled manner.