Why?
Because having the random status effects isn't very tantalizing when the non-status variants kill things much faster. It's like you've almost crafted a final flourish, but instead randomly deal status and have less damage. But we don't want more damage, because that's already in the game as final flourish.
Instead, we should try to emphasize the status effects, and what they're good against. Flamberg has another problem- fiends and beasts resist fire, which is flamberg's jam. The weapon would be far more effective if it dealt damage to some sort of mobs that were actually weak to fire.
The easiest fix
Make them more of what they already are, differentiating them from the other line, and have a goal in mind.
Reduce damage by a bit, increase attack speed.
Significantly higher chance of giving off the respective status effect. More likely than not, to give off the respective status effect.
Flamberg becomes shadow damage; something of a slime killer.
But its damage over time