Especially amongst first generation immigrants like myself.
Any teacher will tell you that parenting is the biggest determining factor in academic achievement, and as a result French immersion is effectively the "academic stream" in the Canadian school system. It also receives additional federal funding which is not available to English programs.
They pretty much learn a second language for free. Why not?
And by extension, English schools that overlap with French immersion schools become biased towards kids with learning difficulties which makes them that much less attractive to high-achieving parents. The TDSB elementary system isn't supposed to be streamed but effectively it is.
(I had two kids in TBSB French immersion until grade 4 when we moved away)