SPECULATION ALERT: Trying to put pieces of the story together. This is a “story” I put together using snippets from the released information.
When the first shootings were reported, witnesses apparently fingered the boyfriend of the dead girl, and someone said they saw him leaving the dorm at about the time of the shooting. They identified his car. This was the WRONG person, note that the early stories kept saying that the suspect in the 1st shooting didn’t match the description of the 2nd shooter.
The police thought they had a domestic killing. Probably there was screaming from the room about cheating, and then the shots and two people were dead. They arrive and people see a dead man and woman, and assume the boyfriend of the girl found her with the RA and shot them both, and then fled. They went after him, and they caught him off campus and were questioning him when the 2nd shootings took place.
Not knowing that Cho existed, and having NO witnesses mention him, they thought it was a standard domestic problem and they had it under control.
If Cho was stalking and going crazy, maybe nobody KNEW he had a fixation on the girl, and so didn’t relate them. And with a co-ed dorm, there are probably people coming and going all the time and nobody saw or notice Cho.