There was a topic called: "Titan's atmosphere is 10 times density of Earth's!"
That topic was about the idea that the more distant to the Sun, the higher the gravity constant of a planet. That at Saturn distance to Sun the gravity is 10 times higher than Earth.
I don't think so about this.
Instead, I think of the following reasons:
Low temperatures found on Titan make molecules to move slowly and more difficult to escape;
Because of the low temperatures and low speed of molecules, the low gravity or escape velocity is not a problem. It is still enough to hold atmosphere;
The atmosphere is covered by a thick orange haze of organics. Such haze, for me, act like ozone on Earth, protecting the surface from UV rays. The absence of UV rays in troposphere combined with low temperatures and maybe intense cryovulcanism or even biological production allow the elevated accumulation of gases. It has more nitrogen in the air because its molecular mass is higher than of methane (so methane escape more easily than nitrogen to space), and because methane break much more easily, unlike nitrogen or oxygen. Or even because methane is used by living organisms...
Titan is protected most time by Saturn's magnectic field against the solar wind;
A very important thing that could explain the mistery of Titan atmosphere: is that the solar wind may be much more weak at 9.5 AU than at 1 AU, so erosion of atmosphere is more difficult and occur much more slowly. To me this is obvious, the higher the distance from a star, the less radiation and influence of star a planet will receive. So the effects of the Sun will be much weaker on Saturn/Titan than on Earth. And that is very important to allow the accumulation of gases to form gas giants. I've even read that Titan only receives 1% of radiation that Earth receive from the Sun!;
Or because the atmosphere is recent and formed only few million years ago, so there's still a lot of gases accumulated.
Well, there must be at least a reason for Titan to have such dense air!
If Titan was the same mass of Earth, its atmosphere would be as dense as Venus or even Titan could be a gas dwarf (small gas giant). This is possible because it would have strong gravity combined with low temperature and lower force of solar wind.
As for other moons of Saturn...
Other moons don't have atmosphere because Titan escape velocity at that temperature is enough to hold a dense air. Enceladus only have some air because water escape from its interior by cryovolcanism. And that atmosphere is certainly temporary, because of Enceladus insufficient gravity to hold on what we call air.
Enceladus also has a great albedo, so the temperature on its surface is certainly lower than on Titan, helping to keep at least a very tenous atmosphere near the surface and that slowly go to space. The same case happen in Triton or even Pluto.