Because it's a section view. It's aligned with one of the section lines. If you want to show it not aligned with either section line you need to break alignment and rotate the view.
Josh is correct in the reason. However,
- You don't need to break alignment. RMB on the section view and select Alignment, then select Align Horizontal by Origin and then select your parent view.
- You also don't need to manually rotate the view. RMB on the section view and select Align Drawing View, then select Horizontal to sheet clockwise or counterclockwise, depending on which direction you'd like your view to be.
As a note, because your cutting line is not orthogonal, it is correct to initially align and orient your section view to a direction that is normal to the first segment of your cutting line. For those small number of cases where the user wishes to align an orthogonal direction, we have the commands mentioned above.