It's the size of the market. The startup cost of making a competitively cheap printer is much larger than the expected revenue. Would you pay $2000 for a whitebox printer? Would anyone? It'd be a cool passion project, but making a legitimate business out of it would be hard.
One big question I have: "is it legal to sell a printer that requires the use of someone else's toner?". If so, then the concern about sourcing toner after the whitebox company fails can be addressed in the design stage.
What I really want to know is why there is no BYOPK whitebox TPM product. That should be easy to make and could dominate the market.