Perhaps the small Mixte requires too much custom configuration for mass production. Smaller wheels, shorter cranks, etc, so they've decided to distribute the frame and not the complete build. It shouldn't be that hard to do, but they'd have to keep a separate stock just for that bike.

In your case, you might choose a custom configuration anyway, drop bars, whatever wheels you choose, Ultegra or 105?

Oh, another thought. Have you considered something like a Bike Friday?

Something like a Pocket Rocket. They should be able to substitute a 406 wheel for the stock 451 if you wish.
https://www.bikefriday.com/bicycles/rocket