Why are Krueger flaps called flaps and not slats?
A flap increases airfoil camber, and can be on the leading edge or trailing edge.  The Krueger Flap is a Leading Edge Flap, the actual generic term.  



The Krueger dude developed the LE flap used on some airliners so his name is attached.  

A slot increases stalling AOA, without necessarily increasing camber, as in the fixed slots used on a lot of light aircraft in the 40s to improve aileron control at the stall instead of wing washout, like Stinsons and Globe Swifts, etc.



Combine the slot function of injecting slightly higher velocity air along the top to keep flow attached at higher AOA, and the flap function of increasing wing chord, together on the leading edge, and you get SLAT.   

Or you could say more correctly, Slotted Leading Edge Flap.

Without the slot part though, it's just a flap, as in Krueger Flap.