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.
I would argue that with the right setup (sensors, gyros, on board computer), an automated system could do a much better job than the cyclist. My guess, is that most cyclists stay well under the 'thrown over' threshold when braking, which could be part of the reason some collisions occur (i.e. too little, too late sort of thing). An automated system could probably get you right up to that threshold, and then use gyros for stabilization (to balance the bike). The reason that this won't happen is because no one is going to install this on their bike (whereas most modern cars already have fairly sophisticated on board computers and sensor systems).
The big thing you're missing is that the rider typically outweighs the bike by a factor of 5-10. Even with all this gear your proposing which would definitely weigh something significant (because ounces are significant in bike weight, and weight is the single most important performance metric for a bike), the physical action of the riders body is going to be the dominant effect of anything that happens.
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.
