I have nothing but a list of whys when it comes to the showcase and custom builds.
For the radiant finishes, I would guess setup time and reproducing it exactly to customer satisfaction would be the biggest things. The 1st one or two that were created for a custom build and returned because they didn't look like the other ones would put the kibosh on that real quick. At least in the showcase, you can see what you're getting first. There's guys on the forum that do showcase only for that reason. How can one blindly order a piece of wood whose grain is unique? Solution, see it first.