Given that Thief and Dancer both have ways to force critical hits, making the weapon skill unable to critical on it's own seems like an intentional move. For better or worse, Rudra's Storm is meant to work in tandem with Sneak Attack/Trick Attack/Climactic Flourish.

Also, just looking at the numbers, it appears single-hit dagger weapon skills are made weaker than most single-hit weapon skills for similar reasons. I think this bit is dumber than eating pudding with a sieve, but it also seems intentional.