But windfoilers have gone wide with not so much lean? All that weight on the wide rail generates a moment about the longitudinal axis that can only be counteracted by the mast generating lateral pressure.

The kite foil configuration has the mast generating a lot less lift, the greater inclination of the wing provides more counteraction to the lift of the sail.

This appeals to my sense of elegance. Generating your total hydrodynamic lift with one foil in the ideal direction has got to be associated with less drag than splitting the lift between two foils at right angles. (Pythagorus and all that - you're generating more total hydrodynamic lift to get the same resultant lift). As well as that the mast is a symmetric foil and won't have the lift to drag of an asymmetric one. ( With the mast generating less lift it can then be made thinner, as most of the forces will now be compression, i.e. less bending )

The only reason I can think of is that a large sail with a wide boom gets a little cramped on a narrow board. Why not have a travelling mast foot? Slides fully leeward to give you max leverage on the sail but not the width to inhibit the ability to crank the foil windward.