So the question is not necessarily why choose CL for a window manager, the question is: why not?!
In quite a lot of cases Programming Language preference decides what WM people end up with, and that's totally fair.
If you are into C/Lua, Awesome might be your thing.
If you are into C, try dwm and i3.
If you are into haskell, then by all means, xmonad might be great for you.
If you are into Python, there is Qtile for that (also quite nice).
For a list see here https://en.wikipedia.org/wiki/Comparison_of_X_window_manager...
At one point I also tried StumpWM out of interest, and though this was a while ago, it was perfectly usable.