I will speak as an educator, because that's my background. In order to teach French immersion, knowledge of the language is critical, but so is training.
You can't just take a francophone from a francophone environment and say, “You are going to teach French immersion”. There are specific training skills that are required to understand the dynamics within the context of learning the language from another perspective.
There is an element of attracting teachers and keeping them, but there is also the element of training so that the teaching-learning experience is successful. It's a specific métier, if you will, to be—and I'm not speaking for the teachers here but as someone who has worked as one.