I refrained from trying to explain Mohr's which shows where the maximum stress in a solid under shear is. It is non-intuitive; unless you want to do a fair amount of preliminary study the explanation won't make much sense, and PPRune is not the ideal place to write a Stress & Strain course.
However: in a horizontal I beam (which is an adequate description of a mainspar) with a vertical force applied to one end, the lower boom is in tension, the upper in compression, and the material in between in shear, trying to stop the upper boom disappearing inwards and the lower departing outwards. The maximum shear stress in the material is at 45 degrees to the neutral axis, and simple shear failure will usually occur at the point along the beam of maximum material stress (obviously). A spar is usually tapered to try to achieve failure at all points simultaneously, and tests prove that no spar is ever perfect, as they usually fail at a single point!
If you can follow the argument in D120's link above, then this is obviously true. If you can't, then the reason is 'because it is'.