Well tbh with you the manuals don't specifically tell you the reasons why, more boeings philosophy these days, do the checklists and don't ask why. My own common sense tells me it's because a high lift device ain't doing its lifting!! so a higher speed required as an insurance policy. Whether it's a simple or more complex matter I'm sure the checklist takes worst case scenario. Have a look at the LD curve of a clean wing vs one with LE devices deployed and it's effects on critical angle, then you can probably take a few good guesses at Boeings reasoning behind the checklist.
Happy to share my simplistic view
