People have a fondness, even a loyalty, for their primary language.
People have a hesitance, even a reluctance, to use a language if they do not feel they have sufficient proficiency in the language to express themselves clearly, perhaps even expertly. Fluency in one language of a family facilitates mastery of a new language in the same family. Introduction of a new language often introduces a new dimension of thought. It's not another language, it's slang. It's not math the way I've always spoken it.