Some diet habits are harder to persuade, and many won't stick with them. So anything is something.

pyre assumes (and I agree with him) that the difficulty (D) of convincing someone to change the habit satisfies:

D(Omnivore => Vegan) > D(Omnivore => No red meat) + D(No red meat => Only fish) + D(Only fish => Vegetarian) + D(Vegetarian => Vegan)

Often intermediate steps are significantly easier to implement than just implementing the end result. It's the same reason for which people tend to break big scary tasks into sequence of many easier ones.