Human Ancestors Were Nearly All Vegetarians- Scientific American

[Scootaloo]

18. There's a trick to being an omnivore...

That trick is that plants are easy to catch, and animals aren't. Bears for instance, are omnivores; aside from polar bears (who have a really high starvation rate, even before this global warming problem) they mainly subsist on plant life. Most meat they eat is either small animals, eggs, insects, or carrion. They'll opportunistically take larger prey of course, but given the choice between picking rotten salmon off the rocks, or risking getting kicked in the face by a moose, bears will eat the dead fish.

Our ancestors undoubtedly followed the same rubric. Even with weapons, hunting was dangerous (witness all the broken bones among neanderthal remains). The kill would attract other critters that enjoyed meat as well - and nothing can ruin your deer like an angsty wolverine, let me tell you.

we undoubtedly ate meat whenever we could get it into our mouths, but much of what we ate probably came from relatively unimpressive sources - insects for instance; locusts in africa, various water flies in marshes, grubs in the jungle... Fish and mollusks would be a major source of protien for shore-dwellers. Rodents, bird egs, reptiles, and carrion making up most of the difference, and every now and then, a large game animal...

My guess is that larger animals were probably the prefered option only when the human group was running low on all the other stuff the critters provided; gut, sinew, hide, bone, and fat all have plenty of uses for a survival society. Going through all that trouble just for a steak isn't likely to cut it, when you can just peel back some bark for some yummy, crunchy roasted beetles.