Compare the 2 extremes here. Look at a wedge head chamber around the entire perimeter of the intake valve and compare it to the chamber around the perimeter of the hemi intake valve. In the chamber the hemi makes a nice 'cone' around the valve. This allows the air to get around the valve and into the chamber very smoothly and very evenly around all sides of the valve. This allows for the near ideal 'pressure recovery' that Darin talks so much about. A wedge head has many different angles around the valve, has shrouding on the cyl wall side and has a problematic short turn. The air flow around the perimeter of a wedge head valve is far from even. Thus more cfm/sq in of valve area for the hemi and the capability of higher rpm.
The canted/rotated valve arrangement takes advantage of much of the high flow characteristics of the hemi without the need for a huge piston dome and large (surface area) combustion chamber.