By way of an example there are a number of very high non-vegetable protein sources which do not get commonly eaten or used as protein sources. Fish eyeballs, goat, donkey, horse, snails (3oz contain 14g protein), insects (crickets have a massive 69g per 100g), shellfish, frog, beaver, porcupine.
Like the hard to digest veg it's a combination of digestibility, availability and stuff like taste, mouthfeel, cookability and so on.
By way of example (cos I asked the company who did our flavouring back when I owned a supplement company) it's perfectly easy to flavour whey and casein proteins with savoury tastes. So any flavour you get in crisps (UK) / chips (US) you can have in a tub of protein. But every time it's been tried it just didn't sell.