We don't know for certain, but since the various satellites may have formed from the same rotating mass that produced Saturn, such a rotating cloud of collapsing gas will have a preferred axis of spin, and an equatorial plane. The orbits of the satellites that also form from this gas will reflect the flattened disk-shaped gas cloud out of which they and Saturn formed. Material that is much inside the outer ring system cannot withstand the gravitational 'tidal' stresses by Saturn, so flat rings are the preferred outcome of all this.