The decision to import it or not is not likely the demand in Canada so much as the cost. If after all fees, duties, certificates, etc. for importation, and a reasonable markup so the business can make a profit, the cost is too high relative to current commercially produced ammo, they may decide not to bring it in. The Canadian market is more than willing to buy up tons of .303B surplus if it's priced at a level consumers can swallow. If the cost to import it means they can't set the price low enough while still remaining profitable, it isn't a good investment.
I have no idea what their actual numbers are but when I hear about a Canadian company like Marstar that deals a lot in surplus ammo deciding not to bring in a certain product that became available on the world markets, I would guess that this has more to do with it than pure demand in Canada.