There are two main reasons to this.
Feral flare has a quite bad reputation right now. Mainly because people often associate it with the "Afkfarming Jungler" since it requires you to kill jungle camps to gain extra stacks (or even the item itself). Then again this reputation doesn't come out of nowhere. At the release of the item a lot of junglers just went up farming for 30 minutes straight and while yes, they were strong after these 30 minutes, their team often lost due to the fact that they were basically playing a 4v5.
The Stats and gold efficiency
Let's face it... Feral flare has horrible stats for most junglers. The item itself is worth 74% of its value and the passive requires you to kill a big monster for 1 extra damage. That means at best if you are only afkfarming in the jungle you might get 3 extra damage per minute and you also need to be farming all day to compensate for the gold income. Also the 10 health on return are not really great. The only thing it's really good for are the high-range ward and the faster clearing of objectives like Baron, Dragon or buffs.
Now I'm not saying that it is a horrible item per se but all the other jungle items are a lot better. Elder Lizard provides you with more Damage through his AD + True damage and Ancient Golem gives you a ton of tankiness and on top of that both items will provide you with an insane amount of HP and Mana sustain.
The nerfs Hit feral flare really hard mainly because you need more stacks and because they nerfed the Attackspeed on the item. Xin-Zhao is literally the only champion where you could still consider buying it if you just look at the stats but then again: You'd have to farm a lot to benefit from the item and this is almost impossible since Xin is among the slowest junglers in the game.
It's better to just go for either Golem or Lizard on these champions while ganking alot instead of buying an item that is useless if you're not afk farming