They weren't the only ones. Clinton used it. It's a generic slogan. The reason it is used it is because it is both simple and effective, and easy to understand.
Every Presidential candidate from the third or 4th one on (the others would have left off the "Great Again" part) might have used it but if a 2020 documentary maker is using it prominently like Matt Tyrnauer did he isn't trying to bring back memories of Bill Clinton.