Why doesn't the cube fall on a face?

When i rotate a cube 45 degrees on the z axis and attach a rigid body(leaving all default settings), the cube falls and remains on the vertex and doesn’t fall on it’s side.

As Robertbu noted, a cube rotated 45’ will not topple when landing on a flat surface because it is perfectly balanced.

To have your cube roll onto a face, ‘unbalance’ by changing the angle of either the cube or the surface that it is falling onto. You can also apply a force along a vector other than straight down (such as a sideways breeze) to prevent this from happening.