Saturn's primordial disc had a lot more water (attracted by Saturn's mass, and less volatile in the colder outer solar system) available to coalesce onto Titan,

Saturn's primordial disc had a lot more water (attracted by Saturn's mass, and less volatile in the colder outer solar system) available to coalesce onto Titan, than in the much hotter and smaller neighborhood of primordial Earth. It's very plausible that Titan has quite a lot more water than Earth. Saturn's other moons are known (by density and gravitational analysis) to also be composed of much water ice, up to 70% or more per moon.