Why is food given to cardinals before & during Papal Conclave closely monitored?

Close monitoring of cardinals' food before and during Papal Conclave dates back to 13th century, when Pope Gregory X allowed one meal/day if cardinals hadn't finalised a pope for three days. This was done to quicken the next Pope's election process. While these rules have been relaxed, cardinals' food is still monitored to ensure it doesn't become tool of subversion.