Ah, Kyurem. The final member of the Tao Trio, languishing in the depths of BL as its brothers fight on in Ubers. Looking at Kyurem's stats, it's not exactly clear why; these stats are absolutely on par with those of the many Uber legendaries. Base 130 attacking stats stand out in particular. On top of this, Dragon and Ice are great offensive types, if together rather redundant.
So, you ask again, why?
Defensively, Ice typing is a severe liability, leaving Kyurem extremely vulnerable to entry hazards and common attacks. This cannot be stressed enough: Kyurem hates entry hazards of all kinds, which makes Rapid Spin support almost compulsory.
Source: Smogon/ Kyurem
Venomoth has never been a top threat; traditionally, the highest praise one could grant it was "not as bad as those other bad Bug-types." Outshining the likes of Butterfree, Beautifly, and Dustox could hardly be called a talent, and Venomoth's decidedly mediocre stats and typing left it as a forgettable Pokemon despite its impressive movepool and abilities.
Source: Smogon/ Venomoth