There are six different points in the US that are considered major destinations for carriers: Florida, Texas, California, the northwest, Chicago, and the northeast. Pair two of these points together and you have a major transport route. The most prominent of these pairings is California (specifically Los Angeles) and the northeast, but transport between all six of these locations is extremely easy.
Less popular regions benefit from being along these routes because trucks can pick up vehicles along the way. For example, if you live in Atlanta and want to ship your car to Seattle it becomes much easier because almost every truck coming from Florida has to pass through Atlanta on their way to the northwest. Similarly, someone in Phoenix will find it much easier to ship to Texas because of trucks coming from California. Nashville is probably the best example of the power of being along these routes. It’s on the way for trucks going between Florida and Seattle, Florida and Chicago, New York and Texas, Chicago and Texas, and New York and California. Because of this Nashville is one of the easiest cities to ship from to almost anywhere in the US. And this connectedness has much larger benefits than simply being able to ship a car easily because these routes are popular with transport trucks of all kinds. Shipment of all sorts of items is possible cheaply and quickly which, of course, is favorable for businesses. By virtue of being along these routes Nashville has sort of become a major auto transport destination of it’s own. Denver has enjoyed a similar situation.
The routes also determine which areas are dead zones. Most of Vermont, New Hampshire, and Maine are basically impossible to ship from because of the nearby northeast megalopolis. It just isn’t worth it for a trucker to come this far north when there are so many cars available without driving as far. A lot of Wyoming, Nevada, and West Virginia are hard to get cars out of for the same reason. It just isn’t worth the extra miles unless the driver is paid a crazy amount. It’s usually far more convenient to meet a carrier.