The thing with trials is that it is rather hard to do in melee range as many bosses have mechanics that you will have to get the frack away as soon as you see it, and being in melee range makes it harder to get away.

Like the first boss in Hel Ra, they don't tell you to stay as far away as possible when he hits 35% hp for the lols, because once that boss hits sub 35% it begin to use an attack that can one hit a dedicated tank holding block, and almost one hit him if he is standing in a veils for the damage mitigation, so melee here will be highly not recommended.

And the 'right' boss for Hel Ra, he drop these meteors on you that can one hit you as well, and it is AoE so you will have to keep moving away from him which waste your dps, while range dps can keep attacking when they dodge.

AA is slightly more melee friendly but even that is debatable considering some of the mechanics of the bosses and tactics that we are forced to use against them.

So those going into trials with melee weapons only may become more of a liability than help in certain bosses, which can potentially cause the team to wipe because dps checks are not met.