They're both strong sith lords to be sure, but Kun's best offense is range force bolts, which Krayt's quite solid against (being one of the few who can bare-hand block force attacks), and sabers, where Exar is certainly good, but can be matched, and when against a foe who can match him, rather than trying to finess his way through, he tends to either outpower or outendure them, counting on his great force strength, which is less likely here. Thus it'll be a long battle, which makes it more likely for Krayt to be able to use DT or such to his advantage.