No, we don't hear about it more because it's not healthful. If it were, the human digestive system would be like that of cattle, but it's not. You can feed everyone carby, fibrous meals, but that's not optimal and not even necessarily better for the environment.
The reason algae is proposed as a way to feed people is that it's not dumping a bunch of carbs, fiber, and incomplete protein down everyone's gullets. Humans don't need to be consuming these things, and there's drawbacks to diets that aren't based on what our species developed to primarily consume over the last 4 or so million years. There's aspects of algae that are preventing it from being a serious food source across the world, but if those roadblocks can be overcome then it can be a game changer because of where it can be grown and what kinds of nutrients it's capable of producing.
In the present, there's no good reason to believe that "everyone" needs to eat more plants and less meat. Meat is a part of the atmospheric carbon cycle, just as are crops. On the contrary to what you're suggesting, and putting aside the fact that fossil fuels are being used throughout livestock and agriculture, individuals can eat whatever they see fit for themselves.