One of the goals for Intelligent Systems with Fire Emblem: Three Houses was to give players a more personal experience with the world and setting. That’s why the support system was revamped to focus more on the player character, who is more of an avatar for the player than a unique hero with their own story.
Speaking with Intelligent Systems ahead of the release of Fire Emblem: Three Houses, director Toshiyuki Kusaskihara explained to IGN why only the player can unlock S-Rank support levels:
“The reason that we have S-Rank endings for only the protagonist has a lot to do with the fact that you’re using an avatar for the game,” Kusakihara says. “We really want the player to feel like they get some kind of satisfaction out of getting to know a certain character the best through gameplay.”
Source: IGN.com Fire Emblem Director Explains How Marriage Has Changed in Three Houses