@Malevolent_King because nobles have extreme classist views especially against commoners. Which is why even though Lindsey is his wife other nobles might have referred to her as a concubine. Because they think she isn't a "suitable partner" and that a "real" partner needs to be someone from nobility
See my lists
ROMANCE:
https://mangabuddy.com/manga-list/171140
BL:
https://mangabuddy.com/manga-list/171146
ACTION:
https://mangabuddy.com/manga-list/171150
https://mangabuddy.com/manga-list/171146
ACTION:
https://mangabuddy.com/manga-list/171150