All other forms of GW apply no matter what health the enemy is at.
Pretty good answers, but lemme flip it around. Why does EC/Mortal Reminder apply on all forms of physical damage no matter what health the target is at? As a counter to 'mages are better at poking' I'll bring up 
"Morellonomicon lasts 6 seconds!!!" Mortal Reminder lasts 5.
Chief Mayonnaise3 votesBecause if it did, poke mages like Xerath would be batshit insane. They could siege people out of a fight with a few skillshots.
The Bearded Bard1 votesAlright lemme change the question around then, why does 
apply on all forms of physical damage no matter what health the target is at?
Certain AD champions are just as good at poking as Xerath is.
Chief Mayonnaise2 votesIma call bs on that. I can only think of maybe 2 ad champs who have the level of range and poke that xerath has and that's caitlyn and varus. Both Cait and Varus have larger cds on their poke. or the enemy has to step on a cait's trap for a headshot proc. Also, the GW on morellonoms lasts for 8 seconds, whereas executioners calling only lasts 3.
Bronze Demon1 votesit applies for 6 seconds, twice as long as EC, 6x that of thrnmail.
The Bearded Bard1 votesIt has a 6 second duration because most mages can't keep up a constant barrage of magic damage, while pretty much all AD can. Even so, I'd take 3 second duration if it could be applied at any time.
The Bearded Bard1 votesMortal Reminder, the item you should be comparing Morello to, lasts 5 seconds.
FlameanatorX1 votesBecause it has a better design than the other grievous wounds items. I think it's pretty dumb that you can counter a large part of some champion's kits, just by buying an item that's completely non-conditional to use.
Also you just bought a high AP item, presumably on a champion with some burst abilities, so if you can't get the target to below 40% with your combo you have bigger problems. Plus, the GW on Morellos lasts way longer than the other items (as others have pointed out).
I think this thread should be renamed to: "Why do the non-Morellonomicon Grevious Wounds items not have a condition to apply their effects", such as being below 40%, or maybe hitting them with at least X number of basic attacks, or whatever.
Ahristocats1 votescuz it lasts for 8 sec
The Bearded Bard1 votesSix. Also, Mortal Reminder lasts 5. One second less and you get to apply it whenever. I'll take that deal.