1. Evolutionary disposition. We do have canine teeth and like underbluewaters said "Animals eating other animals is the basis of much of life on this planet.” This is also alluded to in oh_sigh's statement "we have all the equipment and desire to eat other animals”.
2. Social conditioning. Many humans have been eating meat for generations. They have been eating it some form since they were born. The society en masse around them does not see anything wrong with it. There is an inadequate opposing force in social conditioning to a balance positive bias towards eating meat. Social conditioning plays a huge role. For example. there is social conditioning and bias against eating dogs or horses. Pigs, cows and lamb.... not so much.
3. Intellectual choice. Unlike other animals, humans are intellectually evolved and we act guided by our moral compass. Furthermore, as the most intelligent species on this planet, we have a fiduciary responsibility to other species that we share this planet with. Not always but often enough, the right intellectual choice is against our evolutionary disposition and social conditioning.
If you take evolutionary disposition and social conditioning out of the picture and view killing and eating other sentient beings purely on the basis of intellectual merit, where does your moral compass point to? When framed that way, I feel that the signal-to-noise ratio is improved in framing the morality question.