The nebula was colder near Saturn, than near Jupiter. The nebula near Titan was cold enough to allow the molecule Nitrogen to become solid.

Thus Titan is probably made of Nitrogen ice, and, because Saturn is giving off heat, it is warm enough for Nitrogen to evaporate to form an atmosphere.

The difference between Ganymede and Titan, is that Titan formed in a region cold enough for solid Nitrogen to form. The difference between Titan and Triton, is that Titan is presently in a region that is warm enough for the slow evaporation of Nitrogen.