Sunspots are regions of strong, tangled, magnetic fields. The "boiling" convection cells, powered by the heat deep within the sun. wind up the pre-existing magnetic field and put energy into the fields, much like winding up the rubber bands on a paper airplane. Two factors make this less effective at the poles: 1. the magnetic fields at the sun's poles are generally radial (pointing in to the Sun at one pole and out of the Sun at the other pole, switching every 11 years or so), so the convection is not effective at "winding them up". 2. The convection cells themselves are modified by the coriolis force, which is strongest near the solar equator and zero at the poles. The coriolis force keeps convection cells relatively small, since the change in corotation in latitude is much larger than the flow speed of a typical convection cell.