The official.answer would be as both leagues finish on the same date then nobody is getting any advantage of time off etc.
Championship finishes weeks before the SPL so the staggered playoffs then .ean by the time the winner of the semi final playoff game is determined the the Spl will just wrap up . Without the stagger the championship team gain and extra couple of weeks off to recuperate and study film etc.
It's a load of tosh though as it just means every thing is slanted toward the spl team from the physical and especially the mental rigours of getting through the playoff scheme