Why do we have a moral obligation to animals who do not understand moral concepts?
Hmm, this is similar to a few other topics on here - I’m surprised there hasn’t been a few replies already. 
First, I think it’s important to think about what defines morals: they are essentially a human contruct, which differs slightly between cultures and religions. Whilst some animals will kill for food or to protect their territory, they do not usually cause prolonged suffering, farm other animals, hunt for sport, test on other animals, or cause a significant environmental impact. And they do this instinctively, without being taught morals in school or church. There are, of course, some notable exceptions (rabbits, various insects…) but this is often because of overpopulation, and the world finds ways of controlling this.
Second, you’re assuming that veganism and animal rights is to do with a moral obligation. This is not universally true. It is also to do with health (both personal, and society’s) and maintaining the world’s natural balance. Though compassion is always a very good quality in anyone 
There are many human beings who do not have the ability to understand moral concepts. Examples are infants, the severely mentally retarded, and elderly people who’s minds have deteriorated.
Nevertheless, we believe that we have an obligation to act morally towards these individuals, even though they themselves do not have the ability to make ethical decisions.
Incidentally, it is worth pointing out that human beings are not the only animals capable of understanding moral concepts.
I think human beings are not the only animals capable of understanding moral concepts.