Titan has active sulfur volcanoes that continuously throw out material into its atmosphere.
Titan is bigger than Jupiters Galilean satellites and thus has a smaller escape velocity..
Titan has a larger greenhouse effect, which helps to trap gases near its surface.
The Galilean satellites of Jupiter are colder, thus less chemically active, than Titan.
Titan formed at a lower temperature, which trapped gases inside it that later outgassed.