Because if they buff drain life then a card you have in your collection becomes useful, whereas printing new cards is essentially printing money. Can't fault them for it, they aren't a charity over there.
Warlock desperately needs heal. So, I think this card is playable. But I am not sure what you could kill with 2 damage. Most of early drops are 2/3's and 1/3's. The best targets for this card are obviously Enchanted Raven and Murloc Tidecaller. And for all other situations, this card is not that good. Because Mortal Coil is so much better to deal with 1 health minions.
If they would print this as a 3 mana deal 3 with Lifesteal, this card would be definitely better. A bash that cannot hit face but has better synergy with spell damage. A little bit disappointed, I must say.