I think (although have no official context) that the goal was to give Kha a way to earn the isolation bonus - clear the minions near an otherwise alone squishy.
However, Kha just used it to poke champions so that he could assassinate them even if they weren't isolated. It also gave him near unparalleled jungle clear speeds and good waveclear - two things that are not factored into his power budget. He couldn't keep it.
Why not just move it onto Q? Well, it is a spammable ability, so increasing the base damage any further is just asking for trouble and it is already really strong when the target is isolated. Adding more power here wouldn't help him - isolated targets are dead anyway.