Because with the way the schedule is, you're going to play as many games away as at home. So if you play an attack centred on mediocre pace but decent/ great spinners in SA, England or here, you'll get dutifully smacked.
WI in the 80s and Aus 00s show you can succeeed on flat/ turning pitches with very good/ great quicks. Even SA's record with Donald and Pollock on the SC was pretty decent IIRC (cbf looking it up). The reverse just isn't the case.