Probably because it's drier at the top and that huge water dish is making it way too humid. There's also no appropriate hides for this species; they like narrow crevices (such as between flat pieces of rock or bark).
I had a Vaejovis sp. in a tank with cocofiber that I didn't let dry out completely, and it spent all its time hanging from the lid. When I switched out the substrate for some that was drier, it returned to hiding at ground level.