I can surmise that the plain versions usually get turned into users and the inlayed / decorated knives more often fall into collections. Collection pieces tend to become aftermarket currency depending the needs or current tastes of the collector, whereas the users get, well, used for years and years. Plus, people wanting to try a Sebenza for the first time might understandably go for the lowest cost version initially, so dealer stock would logically get depleted in these items first.
I would be curious to see what CRK would say about the number of plain Sebenza's produced vs. the number of decorated/inlay versions. I would suspect less of the former type, leading to greater scarcity of plain ones from the onset.