You might ask why. Well here’s my answer: Mesmers use weapons the most unique way, at least in the case of the Greatsword. I have noticed that they only use the “elegant” weapons only, but if you think about it, The hammer is an epic way to unleash mindblowing crowd control effects with illusions, or if you want to go full crazy, you can just add healing effects to it or something that actually diffecenciates it from the other hammer classes.
I play mesmer from the very start, and always thought that a hammer would be a fantastic weapon for illusion magic, with a phantasm that knocks down your foe, or an aoe effect with an ethereal hammer slamming the ground pulling all enemies to the center, or any other crazy idea.