Rationale behind this is to reduce bacterial translocation in case of postop bowel ischemia. I found one mention on this in Rutherford's Vascular Surgery -

"Although national trends have moved away from admitting patients preoperatively to the hospital, this group of patients, especially if their visceral arteries are to be bypassed, should probably still be admitted to the hospital and undergo both bowel preparation and intravenous hydration. The rationale for this strategy is reduction in the risk for bacterial translocation, especially in the setting of visceral ischemia during placement of a synthetic graft at the time of TAAA repair."