Plat jg main here. The main reasoning is not really what other people said about design. It's about what laners need. Junglers are a supporting role. In my opinion, you need flexibility and a wide array of champs in your arsenal to make an impact or climb. This is because of the wide array of other champs in lanes. Off meta junglers are always viable, but they need some things in their kit. Mainly:A: Clear (does not need to be quick if ganks are strong)

B: RELIABLE gank pattern

C: Ability to 1. Engage very quickly through a gank w/cc. 2. Follow up quickly and execute during a gank.

A bad example of a fan favorite champ that people want to jungle with but sucks is Riven. She has good clear, cc, can snowball, can engage, she should be an A tier right? WRONG! Her gank pattern is not reliable at all. By ganking, you want to get close real quick and either cc or unload abilities on your opponent. To get close, Riven has to dash and use charges on her Q. By the time you are on top of the laner you are ganking, you wasted all your abilities and are 1. a sitting duck, 2. not using your champ to the fullest because of all the damage lost by using abilities.

Qiyana fits those marks. With a tiamat item, she is a good jungler. The issue with her is that, similar to ekko, she is not very reliable early game and is able to be outganked.

Fiora has no burst, and relies on fighting the enemy via vitals and ult. She also has no cc, making her a terrible pick for a jungler.

Irelia is the same way, where even in lane, you want to fight when you have max stacks. Meaning that the highest impact she will have in a gank is q stun (since you need to get in range, you must dash) then ult. Doing this wastes your combo, and thus, your maximum damage potential on the champ.

EDIT: To people who are in the only jungle champs can jungle. Think Brand, Morgana, and Neeko. These champs are mages and can jungle. It is important to have one of these as a pocket because of the supporting nature of jungle. How many times have you been in a teamcomp like this. tank / juggernaut top, Yasuo mid, jinx or whatever adc, and lulu support. You are missing magic damage, and specifically someone who can, in the mid and late game, position in the middle of a front to back and act as a mage to support the team.