the reason is chaos dmg is not good... 3 seasons so far and this season is yhe worst for it. all the good chaos builds were nerfed already and unlike elemental dmg there arent as many options support gem or node wise for scaling it high.
The one exception to this is Blood Mage who gets base 15% crit chance for all spells but then at the emd of the day that still only works well with crit based blackflame covenant.
I've wasted about a mirrors worth of divs this league trying to make a chaos build on lich and infernalist it just cant compete. Works for campaign but past t14 no go. Also if it takes that much currency for a build to be viabble is it even worth it you know.
I hope u succeed with something though if you do let me know!