It is not definitively known why the ancient Egyptians stopped building pyramids. Well-known structural engineer and Egyptian construction specialist Peter James theorizes that the Egyptians quit building pyramids because they quickly began showing cracks and imperfections. James believes that these imperfections in the pyramids are caused by what the Egyptians thought were improvements in construction techniques.
The limestone that the pyramids are made of expands and contracts in the changing temperatures of the desert. As the limestone expands, it causes the stones to move slightly. Over time this movement causes the stones to shift position and grind down. As the rocks move, the cladding, the smooth coating on the outside of the pyramid, begins to crack and eventually falls off. The Red Pyramid and Great Pyramid, built later than the Bent Pyramid that James originally studied, may have cracked earlier because they were built with no voids between the rocks which means that they could not absorb the movement of the limestone, causing the cracks and the pyramids to not be as visibly appealing to the ancient Egyptians. James believe that this may be a reason why the Egyptians abandoned building the pyramids and moved their burial grounds to the Valley of the Kings.