Because the requirement filters out most of the market before anyone reads the job description.
The arithmetic
Roughly one in five Canadians speaks both English and French. Outside Quebec and New Brunswick, the proportion is lower again.
So a bilingual requirement removes the large majority of your local applicant pool at the first line of the posting.
Where it bites hardest
In provinces with small francophone populations. Statistics Canada found that outside Quebec and New Brunswick, one in ten establishments or fewer required bilingualism — and among those that did, more than one in six expected difficulty hiring.
Small demand, smaller supply.
What employers usually try
Widening the geography, raising the wage, dropping the requirement to "preferred." The first two increase cost; the third means the role goes unfilled in the way that mattered.
The question worth asking first
Does the job actually require French, or does the worker?
If the role genuinely serves French-speaking customers, the requirement is real. If the requirement exists to access an immigration route, the situation is different — under Francophone Mobility the language of work does not need to be French. The worker must speak it; the job need not use it.
What changes the supply
Looking outside Canada. The francophone populations of Morocco, Tunisia, Côte d'Ivoire, Cameroon and Mauritius are large, and Canada's own guidance points employers toward them.
A bilingual posting that reaches those markets is not competing for the same scarce local pool.
FrancoBridge publishes one job post across six French-speaking markets.
This is general information for employers.