Because we don't ban things that are not problematic. Brightpowder is a nuisance at best, and it will be an inferior item choice >90% of the time. The ban is as obscure as banning Quick Claw, which we haven't banned because it's not game-breaking and has virtually no impact on the metagame.
If we ban things because of mere annoying luck, that would certainly justify other unnecessary bans such as Serene Grace, Super Luck, and Sand Veil / Snow Cloak, lolz. If we want to adhere to the rules of banning, we should lift the ban on brightpowder / lax incense. The ban was a side product of Sand Veil Garchomp anyways.