Because a cyclist needs to do very specific things during an emergency brake to avoid being thrown over the handlebars, so an automatic braking system would probably injure / kill the cyclist.
Furthermore, a bike is a lot more agile than a big hunk of steel plodding in a straight line. You can shoot through a tiny gap that a computer would not be able to detect you were aiming for. There is no way to have the necessary safety margin without being totally unusable in practice.