There are many other methods of atmospheric escape a magnetic field can't protect against. UV rays, thermal escape processes, 'freezing' of the atmosphere into a planets crust. A magnetic field isn't the "big factor" in terms of whether not a planet has an atmosphere or not. Look at Venus and Titan, the two other places in the solar system that aren't gas giants and have atmosphere's besides Earth (and Mars). Neither of them have an intrinsic magnetic field and they still have substantial atmospheres.