Why did patricians a set of laws called twelve tables?

Answer 1

The early Roman Republic had no written laws - knowledge of the laws resided with the aristocracy, so anyone who wanted legal advice had to go to one of them. This put the Patricians (hereditary aristocracy) in a position of additional power, as the Plebs (not members of the Patrician families) thus became Clients of the these Patrons - the patrons didn't charge fees like today's lawyers, but instead expected political support and financial support when needed.

As the Plebs gained more political influence, they also wanted to know what the law actually said, so after much pressure including the Plebs walking out of the city, the Patricians agreed to write them down. This first writing of the law was done in twelve tables of laws, hence the Twelve Tables, finally enacted in 449 BCE.

These laws were subsequently amplified as further written laws were progressively enacted over the centuries by the Senate, which had an increasing proportion of Plebs in its ranks.

The civil wars of the Second and First Centuries BCE took its toll on the Patricians, who ceased to be the dominant political force and the title became honorific by the First Century CE.

Answer 2

The patricians did not pass the Law of the Twelve Tables. Laws were passed by the popular assemblies. The patricians agreed to plebeian demands for a set of laws and the setting up of the commission of the decemviri (ten-man commission) to compile the laws, which were then debated publicly and amended. The plebeians wanted a set of written and published laws to have agreed on laws which were also published to have clear rules and to have legal protection. Prior to this laws were secret and this left room for abuse. The plebeians wanted protection against abuse. The patricians agreed to do this because of pressure by the plebeians.