Hi! That is a great question, and one that puzzles a lot of solar scientists. We think that sunspots come from deep within the sun, and pop up to the surface because they are buoyant, like a hot-air balloon. The weaker fields are less buoyant, and even though they rise also, they get torn up by turbulent motions within the sun before they reach the surface. So, only the strongest fields survive to be sunspots!