Jarvan and Vi are primarily bruisers. They have assassin qualities, but their primary role is that of fighter. It makes sense that they are tankier and more survivable than Kha'Zix. I don't believe it is fair to compare fighter assassins to assassin fighters. They are simply not the same thing. You also compared Rengar and Kha'Zix, so I will focus on that, especially since they share the same primary / secondary roles.
Their passives are not the same. Rengar's passive is a utility passive that gives him his unique mobility. Kha'Zix's passive has utility within it, too, but it also has damage associated with it. At max level, that's an extra base damage of 190 magic damage. In my opinion, this is what makes Kha'Zix's ganks scary especially from in lane. The same goes for Rengar.
Kha'Zix has a potential 400% bonus AD scalings on all of his abilities to an isolated target. Rengar has a potential scaling of 150% AD on his empowered Q, and bonus AD scaling to 70% on his E, regardless of empowerment. Without empowerement, that's a 120% AD scaling at max level on his Q. To compare, Kha'Zix's Q without an isolated target and evolution, deals 120% bonus AD in damage whilst also having higher base damages and a lower cooldown at every rank. As an example, both of these champions easily reach 400 AD at full build. Kha'Zix's base AD reaches 108, so 292 of that is bonus AD. His Q at max level would deal 170 + (292 *1.2) = 520.4 to a non-isolated target. Rengar's unempowered Q will deal 150 + (400 * 1.2) = 630. Keep in mind: this is not counting Kha'Zix's passive which can be activated multiple times. Let's look at isolation, evolution, and empowerment. Kha'Zix's evolved and isolated Q will deal 221 + (292 * 2.6) + (10 x Kha'Zix's level (18) = 939.2. Again, this is not counting his passive. Rengar's Q will deal 240 + (400 x 1.5) = 840. Kha'Zix will be able to continually pump out that same number on an isolated target. Rengar's burst is much more situational with empowerment. However, we can't forget that Rengar will deal more damage in autos, too, because the empowered Q has an attack speed steroid and AD steroid. These abilities do very similar damage at end game, and it's hard to say which could do more damage.
Both of their W's are sustaining mechanics whilst also being an aoe form of damage. However, Kha'Zix's is physical damage with a decent ratio and a slow. For this, Kha'Zix will always have the superior damage between the two skills. This ability also has a shorter cooldown. On top of that, Kha'Zix does not have to sacrifice clearing speed for sustain like Rengar must. This means that Kha'Zix will be able to heal and clear just as fast. Taking a look at the math, the most Rengar will ever be able to heal is 50 * (25 * Rengar's level). At 18, this would mean 500 health. Keep in mind, though, that's at extremely low levels of health. Usually, I would use Rengar's heal to stay healthy enough for a gank so it would be healing considerably less. Let's bring up that Rengar has a resistance steroid, unlike Kha'Zix who has a ratio on his W. I would argue that these two skills are a lot of what breaks their similarities. In a team fight, Rengar is always going to be tankier. Kha'Zix is going to deal more damage, though. In the early jungle, I would argue that it's Kha'Zix who actually has the better sustain due to the lower cooldown, being able to spam it more often than Rengar, and the fact that it scales with the skill level itself (not champion level, like Rengar).
Their E abilities can both be gap closers, but Kha'Zix can also use his to run away and outplay, just like Rengar can use his passive to outplay. Rengar's E is what I think makes a huge difference in assassin patterns. Kha'Zix might have more damage, but the sheer hard CC of Rengar's E is the reason why I think people pick him. The reliability of an instant root, instant engage is hard to beat. Kha'Zix can only slow them. He (it?) only has soft CC, albeit he has more than one ability that gives him it.
Their ultimates are both utility abilities. Kha'Zix's ultimate will proc his passive, though, while Rengar will generate ferocity passively from his. It is possible to be able to use up to three empowered abilities from Rengar's ultimate.
To me, they both seem fairly balanced. Kha'Zix will always be able to pick better than Rengar, because of his isolation mechanic. He also has more damage on all of his abilities and much lower cooldowns on those abilities, especially at early ranks. Rengar is more durable and a better team fighter, due to the durability he gains from his W. I would argue that Kha'Zix has a higher skill ceiling than Rengar, because of the sheer amount of outplay potential in his kit.
Now, about your question, "why no love for Kha'Zix?" Well, is Kha'Zix in a bad place right now? I do not think so. His numbers match up to a very popular pick, and he fulfills the assassin role better than Rengar (imo, since he pumps out more damage). I've played against many Kha'Zix jungle in the pre-season. I know it's possible that he can survive in the jungle, and have an impact. So, if he can have an impact and he's not in a bad place, why isn't there love for one of the most popular junglers of season 4? My answer is twofold. 1) the current meta revolves around very mobile champions that have a lot of dashes. Dashes are not affected by soft cc (slows). Kha'Zix uses slows primarily as a ganking tool, along with sheer power. Can you think of anyone who has a hard CC and comparable burst to Kha'Zix that also fills the assassin / fighter jungle role? If you guessed Rengar, you're right! His assassination pattern is much more reliable in the current meta. 2) Kha'Zix, to me, excels at invading the enemy jungle and getting a quick kill on their jungler, and then waltzing out safely. Counter junglers took a really hard hit with the current jungle changes. It is a high risk, low/moderate reward that doesn't really warrant taking the risk anymore.
So, my answer to "y nu luv fur bug" is that the bug is not the problem. It is counter junglers that are in a bad place, and this is what Kha'Zix does best. His assassination patterns aren't reliable enough to be picked consistently either. So until either the meta changes or counter jungling is made to be more rewarding, the bug won't be top tier. I still think he is a good champion to play in certain compositions against certain compositions. Picking him in blind, though? I wouldn't say it's a great idea.
If you're having troubles getting him to survive in the jungle, I'm a jungle main and I can help if you like.
P.S.
J4 / Vi compared to Kha'Zix is not really fair, especially since their ratios aren't actually bigger and their cooldowns are like hella long. More of their damage comes from armor shred and % health damage, anyways.
. After this item, buy 
and then buy
. Contrary to popular belief, Muramana Kha'Zix isn't "trash" or obsolete anymore, even in light of massive nerfs to W.
or 
. I personally prefer Reaver to hit 40% CDR.
or 
on Kha'Zix, because it doesn't help at all because resistances suck right now and it only delays your death by 0.5 seconds.
