Because ring gravitational lenses only happen if the lensing galaxy is EXACTLY along the line of sight between you and the distant galaxy/quasar. If it is off by even a few arc seconds, the ring degenerates into crescents and multiple images depending on the exact distribution of mass within the 'lumpy' lensing galaxy. To get a ring, you need a lensing galaxy like an elliptical galaxy with a smooth, symmetric mass distribution. A spiral galaxy is so internally lumpy with arms and dense nucleus, that I believe it gives multiple images and no rings. Also, entire clusters of galaxies have a roughly smooth gravitational field so they are also observed to give arcs and partial rings.