I have a theory that meteor collision(s) started the outer crust spinning at a slightly different speed than the inner core, which gives us our magnetic field.
And the ‘springy’ action as it oscillates back and forth gives us the CHANGING magnetic field
Mars might have had the same conditions, but after friction caused the oscillations to die away (like a swinging door slows and stops), the magnetic field went away and the solar winds evaporated all the water off the surface.