The flow over a cylinder is considered to be two dimensional, whereas flow over a sphere can be considered as three dimensional. The flow over a cylinder is moving upward and downward, however in sphere apart from upward and downward motion, it is also moving sideways. Since it has an extra direction, there is a lower speed when flowing over sphere. Therefore free stream mach number is higher in order to have a sonic velocity over the sphere, means a larger critical mach number.