The Galilean moons of Jupiter are similar in size to Titan and are also protected by their parent planet's magnetic field. How come only Titan is able to maintain an atmosphere?
Mass obviously has something to do with it. Only Ganymede and Calisto are comparable in size to Titan, and they must have had atmospheres at the time of their formation, 4.5 billion years ago. Your question therefore becomes: why did Titan hang on to an atmosphere while Ganymede and Calisto did not? Another factor is distance from the sun. Titan is much further away than the Galilean moons, therefore colder, which helps it retain an atmosphere. The most important factor of all is probably the composition of their respective primeval atmospheres.
The primeval atmospheres of Calisto and Ganymede were probably similar to those of Mars, Venus and Earth, largely CO2 with a small percentage of nitrogen, methane, ammonia and water vapour, as well as a dash of H2 and He4. They all had oceans of liquid water. Being much further from the sun than Earth, the CO2 and water vapour soon froze out, while the remainder was eventually lost into space, assisted by the solar wind. The same thing happened on Titan, but for some reason I am not able to explain, Titan had a much greater proportion of nitrogen in its atmosphere (currently 97 percent), and nitrogen does not freeze at the prevailing temperature of Titan. Another thing which is hard to explain is how Titan managed to hang on to so much methane. The low freezing point of methane which prevented it freezing out in the same way as CO2 and water vapour is only part of the answer.