Answer:
Because 12 table law was an effective law system
Step-by-step explanation:
The twelve tables law was an effective but harsh system of law back then. All citizens were able to see those 12 bronze tables and they couldn't change it since it was written on the stone.
Those 12 bronze tablets were having law from everything. For example: Land rights, family rights, trials, debts, punishments and that's why this was the perfect foundation for Justinian's code.
Justinian's law had these 12 tables Roman law as the foundation for Justinian's code. His code was in one document and he made some changes. He simplified it and got clear of some laws that weren't relevant anymore. Also, he added a couple of laws.