People estimate that this item has around a 2% drop rate. I know that I've personally only seen 1 mirror drop in the 40+ times I've cleared the fight.
Considering its rarity as well as the difficulty in winning the mirror against 7 other players, the Tidal weapons are out of reach for a majority of players. Not only that, the upgrade that the mirror provides is not worth having to clear the fight hundreds of times when you could get something from Second Coil quicker and more reliably.
I think a slight increase in the drop percentage would be to everyone's benefit. If not just a flat out percentage increase, maybe offset it by making it so that when it drops a mirror, it doesn't drop a weapon.