There are two factors involved in why there are no herbivorous snakes:
1.Their digestive tract.
2.Their teeth

Strangely, while having a particularly large body, snakes have one of the smallest ratios of digestive system length to body mass among vertebrates. The majority of vertebrates have convoluted intestines, which implies there is a lot of coiled-up intestine packed inside. For instance, a 6 foot tall person’s intestines are about 30 feet long. In contrast, a snake’s tract is shorter than its entire body length. The body is so narrow that there isn’t much room for coiling. It takes a while for the cellulose in plant materials to degrade in order to obtain the most nutritious value. A 6′ snake with a 5′ gut could not fully digest food, hence it was essentially a herbivorous snake.

You may argue that an animal that a snake eats stays in the snake’s body for a very long period. True. Yet, a carnivorous meal has far more mass than a vegetarian dinner. You, for instance, purchase a sizable bag of spinach from the store. It’s stuffed full of leaves. It takes up a lot of room in your cart and is maybe 12″ x 8″. As you start to sautee that bag of spinach, it abruptly shrinks to a fist-sized green blob. Plants are not particularly bulky. Hence, after it is compressed, a snake that consumes leaves one at a time isn’t getting nearly as much food.

Plant material cannot be broken down by a snake’s teeth. Molars, which snakes lack, are essential for pre-digesting plant stuff. And as I just mentioned, plant matter digestion need all the assistance it can receive, which molars provide.

We won’t see any hipster vegetarian or vegan snakes as a result of all this.