Alllllright....here is my guess...hehehe

The 727/737 wings have a slightly higher swept back angle near the wing root....you want the inboard part of the wing to stall first and the thickness of the wing in that area makes the 3 krueger flaps the better bet inboard and the outboard 4 slats help delay the stall....

The Boeing 707 initially did not have kruegers to my knowlege and the leading edge slats were something Boeing tested for the 727 and then 737 (similar wing profile) after the inclusion of the kruegers on the 707.  Wing shape and thickness would be another reason for having a krueger flap (thinner vs thicker leading edge slats).  The inboard part of the wing is at its thickest and you would have to have alot more wing slide down vs fold out.   ;)