Basically, the answer to that is they stopped at around level 100. Why? Not sure what answer the devs gave for it. Maybe it was making rangers and assassins too powerful? lol!
I don't recall if they ever gave a cogent reason for it. Mostly, people stopped using them because they didn't work and became useless. The "upgraded" versions of poisons from one xpac to another simply didn't work at all.
Poisons still exist, but they do not scale well with 100-120 content. I still use them with my brigand because why not?
At this time there are no new damage poisons like caustic or hemotoxin which the rangers and assassins have AAs to enhance. They have not been upgraded since AoM. There are current poisons of a different type, however. Marked Target decreases combat mitigation of target. Ignorant Bliss increases physical damage to target and decreases your threat to target. Warding Ebb increases noxious, elemental and arcane damage to target. And the last one is Acidic Blast which increases physical damage to target. These four poisons stack and are still very much used. They are upgraded each expansion.
