As far as I know, the detectors need to be able to interact with the photons, which redirected(or consumed) the outer “branches” that were landing in the outer slits. This left the only two slits untouched. It shows the fallacy of using detection equipment without considering their impact on the environment or experiment, especially when the extremes of our physical world are being tested. In the experiment, the detection equipment, or sensors, were placed in the two slits.