Because you have to draw the line somewhere. For instance, which of these would count:
- Warlock pushes target over the cliff
- Warlock uses suggestion to convince target to jump off cliff
- Warlock uses suggestion to convince 3rd party to push target off cliff
- Warlock cast an illusion concealing cliff, and target walks off
- Warlock convinces target to camp near cliff, knowing there is a ogre living nearby who likes to throw sleeping adventurers over
You are welcome to rule on those however you like, I won't argue. Some of them make my brain hurt though, so I'm inclined to rule you get the benefit only when they are killed by damage you did directly, via either an attack or a spell.