Very true that turboprops are more efficient than jets.
It's probably a marketing decision. The marketing men probably think that customers with a small enough willy to want a jet will want a JET
But I think the D-Jet, with a FL250 limit, is just plain silly. The Cirrus jet, they told me on Saturday, will go to FL270 so they don't have to do RVSM.
The problem is that the low end TP market is already well covered by e.g. the Jetprop. You can pick up a good one for under $1.2M which is certainly cheaper than any of these jets.
And the VLJ market is quite small. The air taxi business model is dead and buried. So you are looking at a mainly replacement market for large piston owners and some TPs. You need to do "something different" to make a mark there.