Why is that? Google provides the answer:
"The bonded ice crystal structure of sintered snow holds up well under compression; it can bear substantial weight without crumbling. Under tension, however, the same block of snow would easily be torn apart with very little force. For this reason, a cross-section of an igloo more resembles a parabolic arch than a hemisphere" Architecture WeekWith a semicircle, the portion of the walls which meet the ground are basically vertical, while the vector of the force from gravity is at an angle to the ground, the two are not aligned and the weight of the snow blocks above pushes out. With a parabola, the portion of the walls next to the ground are aligned with the force pushing down down.
That paragraph was a struggle--too bad I can't go back to high school math to refresh my comprehension of vectors, etc.