I think they have gone for turboprop for a few reasons:

There's an economy of scale with turbofans to some degree. Large high bypass turbofans are extremely efficient, particularly at their cruising mode.

Smaller regional aircraft have more of their fuel consumption dominated by takeoff - the cruising altitude portion of the flight is smaller in proportion.

When doing something experimental, do it on something small to start with. Then scale up when you've got it working.