They are cheaper
A bicycle costs way less than a delivery truck. And that’s just with upfront costs. Think about the costs associated with managing a fleet of trucks; there are maintenance fees. If the trucks get damaged you have to pay to get them repaired. You have to pay an obscene amount for gas (and trucks devour gas like it’s free).
With a cargo bike, first and foremost they cost nothing to use; you don’t have to pay for gas. Secondly, the cost of bicycle maintenance and repairs are a fraction of what it would cost on a truck.
And hey, with all this money the delivery companies will be saving by switching to bicycle couriers, maybe, they’ll lower their prices! A guy can dream, right?