Because they lack all kinds of substances that meateaters get plenty of.

I'm a vegan, and I'm surprised this is downvoted, because it's true. There are plenty of things that you need to be careful to not become deficient in on a vegan diet: protein (in appropriate amino acid balances), creatine, b12, the good Omega 3s (from algae preferably; flax is missing DHA entirely), calcium, iron, to name a few.

You're fine if you take daily algae oil, multivitamin, iron supplement, calcium, and creatine (without going overboard on any of it) and making sure to get lots of protein from balanced sources (eating a ton of soy is actually not a bad idea, because it's a balanced protein), but I wouldn't recommend somebody who doesn't want to have to worry about their nutrition to go vegan. Enough omnivores are already deficient in vitamins and minerals, and veganism only makes it harder.