What best explains why wholesalers can offer lower prices to larger customers?

In a Word: profitability. The transaction cost per unit is less for a large order: Compare an order for 100 units vs. 100 orders for 1 unit. You could pack, ship, bill and process payment much more quickly for 1 transaction of 100 units vs. packing, shipping and billing 100 individual units.

The advertising expense to generate 100 customers would be greater than the cost of acquiring one large customer.

The wholesaler can also avoid the costs associated with retail sales: a store is more expensive to buy/rent than a warehouse. Retail requires more employees, longer hours of operation...higher overheads in general.