Because You Begged Bugged Challenge. 

I’m wondering if it’s actually bugged or 343 purposely did this, because I noticed after a few tries where I missed that I wasn’t dying at all when hitting the ground. Not even fall damage

Honestly it seems like that sloped area just doesn't do fall damage sometimes, which is a quirk of the engine probably. I think Bungie intentionally put the elite where it is t make this achievement way harder than it needs to be

pmsl 343 do you even test things before you launch them?

I tried doing it too lol

havent had any luck either

I don't know if this will solve the issue for anyone else, but after resetting the level to Legendary Difficulty with No Skulls On, I was able to complete the challenge then.

I don't think the challenge specifically is bugged, as the achievement from OG Reach was always picky as well, as Reach is inconsistent on what a fatal fall actually is

got mine to work. was on legendary and i started the melee by the elites head and it gave it to me

Hasn't worked for me, really stupid

Dum dum doesn't even know about 5 second fall damage immunity after a checkpoint