If the chisel was ground at 90 degrees it would wander all over the place.
The effect of grinding it at an angle favouring the corner of the mortice is that....the corner is the lead cut and once that is started the balance of the cut is going to follow the "lead" of the corner, without straying from the corner.
The lead corner cut is going to give way to a slicing action across each face of the mortice rather than a "forced" corrective cut across each face simultaneously, which would give rise to the chisel following the grain and being very difficult to control.
Hope this helps
This is probably not a correct engineering answer but hey it is 02:30 in the morning!!