Lore-wise I doubt a 2 hand weapon for psyker is possible, but only fits for Grey Knights with their Nemesis Force Halberd, sword, hammer, etc. But the problem is; they're for special occasions where they're deemed important to the mission, and seeing we're playing as psykers, we're most likely only going to use 1 handed melee weapon, due to the fact that they use their psychic powers to kill their enemies, and going basic now: It's warp powers they're using. 

And seeing that the warp is basically dangerous on any level, it's better to use the warp powers than using a 2 handed weapon, as weapons using warp powers needs to be prepared for such powers that isn't exactly... 'Pure', I'm not precise in my lore for Warhammer 40k at all, as it's vast and such a fanboner it gives (to me at least)... But there's also talk of rarity of materials (not ingame, but lore)... So you have to take in consideration of that, too.

Since Psykers are crappy melee combatants, I don't miss more melee weapons at all. I can kill a dozen standard Mobs 300 points above my power level with a single AOE, why should I go into melee and take a Minute... and lots of damage.