I feel like they did it to be able to justify Bonnie giving it to Damon instead of using it for herself/someone she loved and also so they didn't have to mention anything about the connection between witches and vampires since they just re-wrote their own canon to say that, actually, witches can become vampires without losing their magic! Like, the whole point of Damon turning Abbie was that you couldn't be a witch and a vampire but now that's totally a thing.