The Visa Debit cards in Canada carry the Interac logo... so although Visa and Interac are direct competitors, they are linked in Canada. I'm talking strictly in the Canadian space, things are different south of the border as there is no Interac equivalent there.
One of the main reasons retailers prefer Interac is the flat rate fee of $0.25/transaction vs. Visa/MC/AMEX which charge a percentage (Interchange fee).