With regard to the definition of a francophone immigrant, research in recent years has shown the steering committee that francophone immigrants most likely to integrate and stay in francophone minority communities are those whose first official language is French. That's why the definition reflects that fact.
When immigrants know both official languages, they usually favour one or the other, English or French.
For the moment, our systems calculate knowledge of French and English. CIC can therefore index the percentage of immigrants who have both French and English, or only French or only English.
It is important to know which language immigrants are most likely to use when they wind up in Canada. This explains why we have a definition based on the first official language spoken. What everyday language will immigrants use?