They probably get lower rates of delivered to private homes
While courier companies usually charge a consistent rate based upon weight, they are usually used for small volume shipments. Large haulers must normally take loading and unloading time and ease into consideration particularly if certain homes will be used as a central hub. Paper is heavy. A private home means no loading dock which then requires that every piece be handled individually. Depending on the type of home and access, a larger truck will often need to be offloaded at the street, likely to a hand cart. All this, regardless of weather.
As nevaagain related above, it appears that WT is also eliminating sorting prior to shipment. Instead of placing each congregation's order in separate boxes which the intended recipient is clearly identified, they are instead combining all orders together and then offloading responsibility for sorting and repackaging to the Literature Servants at the receiving end. I guess they are called a 'Servant' for a reason.