A sharp heeled shoe will have a little area in touch with the sand. It exerts a much greater pressure on the sand. Hence, it is difficult to walk with high heels as the heels sink deeper into the sand due to high pressure. On the other hand, for flat slippers, a larger amount of area is in contact with the sand. It exerts a much lower pressure on the sand. Therefore, it is much easier to walk on the sand with flatter shoes than with high heels.