Why are direct marketing channels possible for some products and not others?

It is not cost effective for certain manufacturers to sell directly to consumers. In many cases, manufacturers are set up to create large numbers of products to sell to other manufacturers in bulk quantities in the thousands or millions like components that go into electronics. Distributors are in business to repackage and sell those products in smaller quantities to end consumers, smaller manufacturers and businesses. Many small businesses can buy 100 of a component, but not tens of thousands of that component.