It’s clear that our stomachs can’t break down most greenery around us, like ruminants with 4 stomachs can. But what makes one leaf digestible to us and another not?
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Two things:
1. Cellulose. As other have mentioned this is the primary reason. No vertebrate can digest cellulose. Too much cellulose can cause bloating, indigestion, diarrhea, gas, etc. Its not good for us. Some vertebrate animals have formed symbiotic relationships to allow them to handle more, such as cows who have one stomach filled with bacteria that helps break it down. But didn’t go down that path.
2. Chemistry. Different leaves contain different chemicals that may serve a useful purpose to the plant, but are not so useful, and sometimes down right dangerous to us.
Plants like lettuce have been cultivated over centuries or millennial to produce more human friendly versions. Variants that have too much cellulose or dangerous chemicals were selected out in favor of more friendly versions over time. Wild cousins may still exist which we wouldn’t want to eat, but what you get in the market has been tamed by human agriculture over time.
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