It could be a multitude of reasons. Signal propagation isn't as simple as: A half mile in one direction I get signal, but in the other I don't. Terrain, trees, buildings are all going to have an effect on the signal.
Further, the antennas used on a cell phone tower are quite directional. So it could be an alignment problem, or the sector that points in your general direction just isn't pointed in a good enough direction for you to get the signal in the location you want it.
Sometime they don't even point towers in all directions from the tower itself. Are you certain there is a sprint sector pointed in your general direction?