Diets are something you do for yourself, your own health. You can go on and off a diet whenever you want. You can cheat every now and then, since a little bit won't do any harm to your health. It's entirely about you.
Being vegan is not about ourselves. It is about rejecting violence towards other animals. It is not something you can go on and off or cheat on. Once we recognise that using others is wrong, we can't go back and we can't only do it part-time. How could we? We'd be betraying the victims if we did.
Of course, being vegan can have great health benefits, but they're just benefits. We should never forget who it's really about.