Because, to be fair, just about every other item range has been covered by the Seraph vendors. That, and I'm sure they'd make a nice alternative to the legendary grade class mods. --- "Don't eat me. You won't like me if you eat me."
It'd be interesting. I've noticed that the legendary mods all boost the first skills in the trees, presumably so you can get the benefit from them without wasting points going down a tree you don't like, but it'd be nice to see a more specialized mod with almost as good bonuses. --- "My show's not a comedy. It's a horrordy. It's called Blood Dumpster." "Your pilot gave me nightmares!"
It's entirely possible that new skills may be added into the empty slots on the skill trees, to compensate for possible extra skill points in a possible level increase to avoid possibly OP character builds.
Maybe it's possible that they may introduce Seraph class mods for possible new skills?