Because if they, as an imam, issued a fatwa while they were going through their PMS, that will make the entire humanity suffer. Similarly, if they took a step during their PMS, that will surely go against the favor of whichever nation/country/community they are leading.
I dont think it would have been possible to take away their power from them only for the duration of their PMS.
bhai sahab yeh pms kay chakar main kiyoun par ghaiay