When discussing common law, the answer invariably comes down to “tradition”. The English common law, which we inherited our own traditions from and much of whi

When discussing common law, the answer invariably comes down to “tradition”. The English common law, which we inherited our own traditions from and much of which is still applicable today, is less a “law” in the sense of a set of decrees from a King or statutes passed by a parliamentary body and more a long line of traditions and community standards that change slightly over time according to a few principles, with the facts and circumstances of the time used to inform the judgement and sentencing. Without a legislative body, you could go on evolving the traditions over time to cover any number of new and unique circumstances simply by using the existing body of law as a guideline. So why 12 Jurors and why must they be unanimous? Because it is tradition, and the principle of stare decisis has kept it that way for a very long time. Can traditions change? Yes, but stare decisis makes it unlikely that any sort of arbitrary changes will be made and upheld. Think of it as the legal equivalent of inertia. The law stays the same given the same facts and circumstances unless changed by an outside force, usually a statute in modern democracies with common law legal systems. The upshot of all of this is to make the practice of law and process of legal proceedings more predictable and arbitration less arbitrary if you’ve done your homework and can tie your case to a previous case, and it doesn’t even necessarily have to be a case from the same jurisdiction if it’s from the same legal tradition and not incompatible with the existing body of law in the jurisdiction the case is within. In contrast, whenever the legislature passes a new Bill and it’s signed into law, you have to study how this will affect new cases going forward rather than simply being able to rely on past precedent. This allows more instantaneous changes to new circumstances but also allows more arbitrary laws to be passed whose effects on people and commerce are not well understood until they are used in litigation.