Many legal terms are in Latin rather than English because of the lasting influence of the Roman legal system on Western legal systems that exist today. The Latin phrases are still useful because their meanings are still relevant in the American legal system.
Many Latin legal words or phrases have been absorbed into English so that many non-lawyers know what they mean. Some examples include alias, alibi, per se, and versus. All of these are Latin terms that have a legal meaning. Rather than inventing new English words that would mean the same thing, the English system simply continued to use the Latin words.