When rengar goes in for a kill, he can't go out. After using everything he has on a squishy carry, he'll become vulnerable to the rest, whilst kha can still dish out a ton of damage and manage to jump out after getting a kill/assist. As well as his ult still intact for the rest of the fight.
I'd feel much more comfortable playing kha, ton of constant damage, decent sustain and has a much easier time escaping from a bad situation, and laning isn't as frustrating as it is with rengar.
Not to mention that kha still has a glimmer of hope if he falls behind, whilst rengar is more either kill everything if fed or not contributing much in a team fight if not.
I don't know, probably just me cause I suck at playing rengar.