At a very simplified level, in these kinds of astrophysical contexts you can kind of think of magnetic field lines as strings, and the particles in the gas are

At a very simplified level, in these kinds of astrophysical contexts you can kind of think of magnetic field lines as strings, and the particles in the gas are forced to follow the strings. The stronger the field, the more closely the particles have to follow those strings. So if the magnetic field is large, they can have the effect of redirecting particles from where they otherwise would go due to gravitational collapse, and slow the process down.