I've got like four costcos in easy driving distance from me. Roughly equidistant too, so other than preference, there's nothing stopping me from going to any of them. Well, one of them is in montreal, so traffic is a bitch. And of them is the "corporate" version of the store, so it bulkier bulk stuff meant for stores and offices.
Yeah, canada has several supermarket chains, both regional and national. But the actual number of companies that run them is miniscule, with each company owning several of the "brands". For example, Sobeys owns safeway, thrifty foods, IGA (independent grocers association. Well, they were once.) Freshco and a couple of others. Loblaws owns the self titled store, provigo, maxi... It's nothing but megalithic corporations all colluding to maximise profits.