Because the problem is that the ECM is supposed to send a signal out for a certain self test, and yours didn't. So, either the ECM is not getting the signal it needs so as to send a signal, or it's bad.
In laymans terms, think of it this way:
Electricity is water. The battery is like a fire hydrant. Turn the key on, and a hose that goes to the ECM fills with water, and the ECM can now send out water where it needs to. During the KOEO test, I notice that one or more of the places that the ECM is supposed to spray with water are dry. That means either the ECM isn't getting the water, or it is getting it, but not sending it out. So, I disconnect the hose from the battery to the ECM, and check it to see if it's got water from the battery. It does. That means the ECM is getting it, but nolt giving it, which means it's bad. By determining that the ECM was getting sufficient voltage, we proved that there is a failure inside the ECM that is not permitting it to send the required voltage elsewhere.
Take care,
~Chris
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