The meta shifted. With the changes to bork, as well as nerfs to most of the reworked assassins(as small as they may be) and other champions. The meta shifted, it still favors Kha'zix and rengar in the jg(although Rengar took some hits to his early clears) and leblanc is still a solid pick mid if you are good with her(so long as you follow the new build because RIP W dmg).
They are still strong, they just aren't favored picks currently. Graves and Ivern are both performing extremely well in the jg, and they handle assassins well. Lulu is the pick/ban support right now, and she cucks assassin's over so hard that mages have a better time against her. The bork buffs allow ADCs to trade favorably with the assassin that jumps them now that it is a solid rush item etc. etc.
They aren't bad by any stretch, they just aren't insta-lock picks(across the board) anymore.