My guess is they chose the foot because the vasculature is very superficial over a relatively flat area with lots of decent sized vascular branches. So, they can tap into the blood supply relatively easily. Also the patient would probably be less inclined (or fully incapable) of fucking with it - you'd be amazed how many patients turn a nearly invisible incision line into a jagged, infected mess, cuz they just can't resist picking at it. And I get it - I'll pick myself bloody if I notice so much as a bump starting to grow on my hand... a full blown ear would demand way more attention, so keeping it out of sight / out of mind was likely a strategic move.