Is there lore implication or some gameplay reason to this? Just wondering.
Because psykers die after getting hit even once? 
Some OP uniqueness. Class weapons tend to have some interaction with the class, and Psyker has the smallest cooldown between dodges so I guess it fits well. To add to this, the basic FS was heavily single target and you relied on the infinite dodge in horde situations while thinning it (it helps a lot).
Because psykers are dodgy bastards!
that should be fixed, they should die twice 