Brig had the worst healing in the game that was dependent on a proc. Her healing was so bad that it was required to run two other Supports alongside her in order to have enough sustain to win teamfights. Her Ult was potent, but surviving long enough to have it without two other Supports picking up the slack was impossible.
That was the reason why OWL teams stopped running Brig the moment opposing teams stopped running Dive, and why her pickrates nosedived quickly in Season 11. Running Brig meant you had to play her in the DPS slot, and unless you needed her to fight Dive, it just wasn’t worth it.