There is a famous rule of evidence in American law: "There can be only one bedbug in a noodle." This is a vivid metaphor: If anyone finds a bedbug in his bowl of noodles, he will never go looking for a second one, but will throw away the entire bowl of noodles. Similarly, although the Los Angeles police obtained a large amount of evidence that could prove Simpson's guilt, as long as one of the pieces of evidence (the socks) was obtained illegally, then none of this evidence would be admissible in court. The jury therefore found Simpson not guilty.
The democratic system of the United States has always been established to prevent the erosion of civil rights caused by excessive power. This is reflected in criminal convictions: "It is better to kill three thousand people by mistake than to kill one person in vain!". The jury formally followed this constitutional principle and found Simpson not guilty, but only criminally.