Stating the obvious I'm guessing, but in case this wasn't rhetorical, it's because they don't want you putting it on your opponent's creature and then removing it. I'm not sure why they were worried about that though, it wouldn't be broken or anything. I suppose it could maybe see play as a sideboard card against linear ramp/reanimator decks, since then you'd be getting a 12+ life swing for 2-mana. Still, wouldn't be broken and it depends on your opponent playing a large creature that you have removal for, guessing they just thought that was a "feel-bad" interaction.
As a card, it's almost strictly worse than the 2B sorcery from AVR that does the same thing (essence harvest), but doesn't require the creature's death. I guess it spreads the mana investment out over a turn if you want to combine it with pump spells. Essence harvest was always borderline playable in draft IMO if you had a nice homicidal seclusion deck, I suspect this will be similar. There will be some borderline cases where your deck has enough ways to enable 10+ life-swings and where the life-gain is relevant against your opponent.
I think the intention was to make positive Auras more viable. Currently, if you buff a creature up with Auras like Deviant Glee or Ethereal Armor , they can just remove the creature and you'll have wasted your auras. With this, you can buff a creature's power as much as you want: if they leave it they're in trouble, but if they remove it they're still in trouble.