Why were Romans not truly equal before the law?

Because Rome was a class-based society, divided into Roman citizens and non-citizens. Only patricians (noble class, wealthy landowners and old families) could be citizens. On the other hand, plebeians (common people) and slaves were not citizens. Roman society was like a special elite club with many privileges and rights.

Roman citizens had many legal and social advantages, like the right to be free, to vote, to make contracts, to own property, to have a lawful marriage, to have a legal trial, etc. Non-citizens and slaves did not have any rights.

Therefore, Roman people could not be equal before the law, until the first laws of equality had been created.