If a LEO assumes miscreants are likely to be at the house, they might be reluctant to make their presence know for fear of drawing fire. I am not saying all LEO's react this way. Some would choose to announce themselves loudly.These are exactly the types of calls from which cops are ambushed. I am not justifying this shooting, just answering your "why" question.
I want to know if they talked to the complainant before approaching the house
In hindsight, the latter approach would probably have averted this outcome. But hindsight is 20-20.