We didn't evolve into ominvores as much as we evolved to survive eating almost anything. We come from herbivorous ancestors, and we became increasingly carnivorous. Because of our lineage, our bodies are not adapted to hunting. To successfully acquire the most valuable nutrition (other animals), we would have to use our brains. The best hunters were the smartest. They were also the most reproductively successful. Fast forward and humankind has become so brilliant from hunting (the nutrition/challenge of meat) that we invent agriculture. Everything resulting from agriculture - technology, societies, and nutrition - is completely at odds with what is natural for us and the world. So successful, we have destroyed natural order and will be consumed by the incongruencies between our world and the real world. Ironic, but that's life.

Anyway. One of the reasons humankind has been as successful as it has is because of how we can thrive in such a diverse range of environments. From complete carnivores, to heavily vegetarian, we can eat almost anything and live. More specialized creatures do not enjoy this advantage, they are adversely affected by slight change in ecosystem. It's not so much that we evolved into omnivores, it's more that we became increasingly carnivorous while also retaining much of our herbivorous traits. It was adaptive to keep them, so we did.