It is my guess each brand gets a dedicated executive and team, and internal politics plays a role I guess. If a certain executive has a "favourite", to call it that way, they might first install them in the posh Sofitel team, and then after clashes with Sofitel management etc., have "SO" elevated to a separate brand and have the person in question lead that. That way, they can justify luxury travel and expenses as "research", and few people can be incompetent at spending money on luxury.

BTW, Mondrian is not ALL-aligned, so it is as good as none. There is a recently-renovated MGallery in Cannes though.