Because "soldier" is a military term and sounds aggressive, while "guardian" sounds like some innocent person defending from aggression. A belated attempt to shift the responsibility for countless people murdered by the senshi while they were pursuing their political goals and consolidating power to rule the planet.
The same tactic was used by Thutmose III, Alexander of Macedon, Julius Caesar, Hitler, Stalin, and every other murderous psychopath throughout the human history. None of them admitted they aggressively attacked anybody, ever. They always claimed they were defending from aggression, and conquered other countries and nations solely because there was no other choice to "protect" and "guard" innocent people.