Dealers stock what sells best in their area. They don't want to load up inventory on trucks that will sit around for months until the right buyer finally comes along. It is all based on sales history and demographics.
The Express is catered to a specific target audience, just like the other trims. Similar to others here, I purchased an Express black edition because it has everything I want while also looking bad ass and didn't waste my money on features I don't want. I hate chrome and leather seats so the Express checks that off. I got the UConnect 5.0, backup camera, heated mirrors... this gives me all the tech features I want. Etc.
You can do a custom order or look at other trims to find what you want easier. If Rambox is important, the Express just might not be the model for you.