I'll tell you why. Because it was a rubbish idea, poorly executed. OK, rephrase: as a standalone idea, it sucked. As a component of something else it might work.
The idea was that you'd find a friend on Facebook and then send them a virtual present. A beer, a cake, whatever. Now you can call me old-fashioned if you want, but if anyone wishes to send me a beer they're welcome to do so - if I can drink it. Likewise cakes. I know I shouldn't but if I get offered a cake I might well eat it; if I'm offered a virtual cake which I can't eat, my reaction is more likely to be "You what?"
People liked the idea initially as a bit of fun. It didn't actually go very far, though. So if we fast forward away from the beginnings of Facebook and to the present day, we find a very different set-up. There's this Farmville game, for example. I'm the first to admit that I just don't get it, but it's popular. You manage a virtual farm, exchange crops and whatever with friends and build up some sort of establishment.
Compare that to "you have just been sent a virtual apple" or something and you realise just how dated the old version had become. Whether it was to your tastes or not, its limitations were becoming apparent.
So the options were, I'd speculate, that Facebook had either to revamp its gift offering completely and make it a load more sophisticated, or accept that other people were using the platform to do it better than was ever going to happen in-house. Sensibly, they opted for the latter.
This means of course Facebook gets other people who're working on Farmaround and other games helping to consolidate its market lead without actually having to do anything. Nice work if you can get it!