Why did Plutarch face difficulties writing about Roman figures?

When writing his famous Parallel Lives, Plutarch faced a significant handicap regarding his Roman subjects: he possessed an imperfect knowledge of the Latin language.1

Although he was a prolific biographer, Plutarch had learned Latin only late in his life. During his time in Rome and Italy, he was heavily occupied with political tasks and the teaching of philosophy, which limited his ability to master the language to the level required for his research. Consequently, while his work displayed extensive investigation and research, he was not always able to consult all of his source materials at first hand.1