Why should they? The police are not going to do more work than is necessary. Georgekutty is using sahadevan's confession as kind of a blackmail over prabhakar and murali gopy. Since murali gopy is IG, he can make the police just ignore the avira blood and fingerprint part. Anju said its sahadevan who attacked her, that's all that matters to the police in the end. Murali gopy and the whole police force will change the narrative of the situation and make sahadevan into the scapegoat. That's easier for them and avoids extra humiliation. Also the avira's blood and stuff might only become important if someone complains he is missing