Those containers can go straight from Houston to Dallas, Fort Worth, El Reno, Wichita, Topeka, Kansas City, Des Moines, Albert Lea or the Twin Cities... all directly on the UP's ex-RI/ex-C&NW "Spine Line." Run 'em up the middle to the latitude you need and they can go east OR west from there.
Also, on the Florida side, Jacksonville port has the CSX, Norfolk & Southern, and a coastline local railroad that runs down into Florida. Plus, I-95, I-10, and access to US 82 to I-75. Can get to all the southeast in 1 day, most of the eastern half of the country in 2 days.
I think that the availability of offloading capacity outweighs the cost of traveling through the Panama Canal to the east coast and gulf. Plus, it eases pressure on the west coast ports.