Easy is definitely a big part of it. It's not impossible to make tasty and nutritious food without meat, it's just not as easy. Especially if you don't use any animal products at all. From an environmental and ethical point of view those don't really differ that much anyway.

Fake meat doesn't really taste like real meat yet, which is one reason why they aren't selling as well as hoped. I think it will be easier to develop fake meats which are indistinguishable from the real thing than convincing the majority of the people to forgo meat without having an equivalent replacement available. Making really really good fake meat is going to be extremely hard, making the majority of people vegan (or even significantly reduce their animal product consumption) without it will be borderline impossible.