the argument that… rests chiefly on the fact that…

This means the reason why… is….

  • Mr Ritter’s argument that Amazon and Zalando can comfortably co-exist rests chiefly on the fact that Amazon is pursuing the more price-conscious shopper, whereas Zalando is after a higher-value, more brand-conscious segment.
  • The argument that higher education should be made universally available rests chiefly on the fact that doing so is in society’s best economic interests.