Answer:
Because 12 table system had every sort of law and that was effective a long time
Step-by-step explanation:
He used 12 table systems of Roman laws because that sort of law had every law in those 12 tables and that included family laws, marriage laws, debt and trial laws, punishment laws, relationships with citizens. People were familiar already with those laws.
Justinian used this system as the foundation and changed it a bit, and also he simplified it in one document and it was called Justinian's code.