Anti-trust laws + a lot of other stuff that used to be kosher but is now super illegal. For instance, look up "credit mobilier". Or consider how Standard Oil

Anti-trust laws + a lot of other stuff that used to be kosher but is now super illegal. For instance, look up "credit mobilier". Or consider how Standard Oil used their size to control railroads and basically make it so their competitors couldn't ship at reasonable rates. As another comment points out, Carlos Slim's net worth is something like 4% of Mexico's GDP. Carlos Slim is pretty much a monopolist though, which helps.