The magnetic field at the poles is substantially different from that at the Sun's equator. At the equator, the magnetic field lines generally do not stray far from the surface. They come out from one point in the surface and go back in again not far away, forming loops. Sunspots are found at the bases of really strong loops. At the poles, however, the magnetic field lines go out to far distances away from the Sun. If they go back again at all, it's far away from where they came out. Sunspots can't form under these conditions.