Maybe this is why vertical grain is used:

I think we all agree that the shear force is at 45 degrees to the wing span. However, this force can be broken down into a spanwise force and a vertical force. With the diagonal force broken into components you can see how the different grain orientations perform.

Lets look at vertical grain shear webs: Here the vertical force, tension or compression, is resisted by the vertical grain. All is OK. The spanwise force, T or C, is resisted across the grain. This is not an ideal condition. However the wood only spans from chord to chord so this isn't so much of an issue.

Now for spanwise shear webs: The spanwise component is resisted along the grain and all is good. The vertical componenet is resisted across the grain, bad things might happen here. The web spans from rib to rib, a much longer distance than spar to spar. Also, the rib grain is oriented in the same direction as the shearweb, so this doesn't really help hold things together either. I can see where the shear web could split or buckle under load easier with the grain running in this direction.

DLBirks