Why is a blunt trailing edge a better stabilizer at hypersonic speeds?
A converging shape at hypersonic speed in a low-pressure medium will produce close to vacuum pressure on its surfaces (hypersonic shielding). A small sideslip angle will only result in a very small pressure difference between both sides.

Contrast this with a diverging shape which produces higher than ambient pressure on both sides. In hypersonic flow this pressure grows approximately with the square of the inclination angle. The same sideslip angle, therefore, will result in a much higher pressure difference, rendering the diverging shape more effective in sideslip.

A more detailed discussion can be found here, page 11-6 to 11-8.

A wedge shape was chosen since it allowed to predict forces and temperatures with greater confidence than would had been possible with a more complex shape.