Jungle is hell because keystones only effect champions.

The mastery system was changed from being a free ball of stats to essentially 29 extremely weak points and one extremely strong keystone mastery. Problem is, none of the keystones help with clearing. There is one keystone that interacts with monsters but it's lategame mastery and it's far weaker than the 2 other options in resolve. Basically, the first clear on any champion that isn't 100% top tier is an absolute nightmare. With the machete/talisman effects being split, if you can't utilize talisman you go OOM immediately, and even then it does little to help junglers who struggle with first/second clear. Powerful ball of stat masteries were replaced with scaling and % masteries or lost effects against non-champions altogether.

By the time you've got your machete upgrade or you've accumulated some health from strength of the ages, you wouldn't even have issues without those at that point.

tl;dr new masteries killed first clear, sometimes even 2nd clear on half the junglers that used to be viable.

EDIT: Forgot to mention Strength of the ages doesn't even restore HP on monsters, only siege minions. You have to kill 30 monsters before this mastery effect is at it's fullest and even then it doesn't benefit junglers.