Maureen is the #1 player in Canada and has been for the last four years. You want the top player from your country playing in your home tournament, case closed. Marie-Eve is head and shoulders better than any other Canadian save for Mo.
Beier Ko has been stuck in the same spot for the last year or two, so she'll make do with a qualy WC. The other MD WC comes down to Wosniak and Dubois, and while Wosniak has represented Canada well in Fed Cup, she's only played 2 tournaments in the last year, so just playing matches in qualifying will give her good experience. Dubois just won a challenger event, which is more than any other Canadian has done this year, so she deserves the other WC.