When I worked as a consultant at Home Depot's headquarters, they often sent out emails from the "Orange Voice" pushing employees to lobby their members of Congress on various bits of legislation and issues. What was very clear was that Home Depot absolutely hated each state having its own set of laws. Even worse were local laws. They wanted there to be just one government for them to lobby for their issues and when one state requires them to have some operational quirk such as customer accessible price scanners with printers attached, the first instinct is to find a way to get the feds to find a way to make that state requirement go away.
That's different and understandable. Everyone wants to have to comply with one set of regulations instead of one set of regulations plus fifty more additional sets of regulations (or more with counties/cities) that require additional effort to comply with and ensure the correct ones are complied with. Just the additional labor costs for lawyers must be real. Add to that the need to have special cash registers for one state, or other changes, and it gets pretty expensive. Hell, Home Depot would probably rather one national sales tax (regardless of whether it's higher or lower than the current national average) over the complexity of the current sales tax code. Just for the operational simplicity.
