I think it has to do with the profitability of doing so. Filler trucks regularly do 250, 500, and 1000 gallon tanks. Those pumps run very fast, so they're able to pump a lot of propane in a very short time. By selling a lot, the companies involved are able to make up for the expense of buying/maintaining the fairly specialized delivery vehicle and paying the salary of the filler/driver, who, in many states, must have a special license to handle propane in this way.
Now take the same vehicle and same driver, and have the person go out and fill, say, 30 or 40 gallons per stop. It's going to take a whole lot of stops to create the income necessary to maintain the business.
:)
Lynn