In the US, we (as a collective) hold sacrosanct the rights of a person against illegal searches and self-incrimination, as well as the 'innocent until proven guilty' doctrine. The idea is that it's better to let a criminal go than to convict an innocent person.
Disallowing illegally obtained evidence also has the effect of discouraging the police from illegally obtaining evidence in the first place and reducing opportunities for corruption. Of course, in this case it didn't work out so well, but you would see a lot more stories like this if sources of evidence weren't important.
Disallowing illegally obtained evidence also has the effect of discouraging the police from illegally obtaining evidence in the first place
Exactly, and arguably it's the only effective deterrent, because courts will be very lenient towards law enforcement officers doing their job, simply out of pragmatism. Law enforcement officers don't all have a precise understanding of the law, there are gray areas in the law anyway, and on top of that, the law is constantly evolving. Prosecuting police officers for illegal searches would be harmful, impractical, and _extremely_ unpopular, so the use of illegal searches and evidence obtained from them would become routine if that were the only deterrent.
Disallowing illegally obtained evidence also has the effect of discouraging the police from illegally obtaining evidence in the first place and reducing opportunities for corruption. Of course, in this case it didn't work out so well, but you would see a lot more stories like this if sources of evidence weren't important.