The earliest signs of terrestrial tetrapod herbivory are provided by Edaphosaurus. Remember by the way, the Pelycosaurs include both carnivorous and herbivores members. This Edaphosaurus has a small head and a big body. Dimetrodon, the other one that turns up frequently in reconstructions, has a relatively large head and smaller body. You take one look at his dental battery and you know that he is not interested in alfalfa sprouts. Edaphosaurus, however, has large grinding and crushing teeth. In older books Edaphosaurus is shown as a carnivore, but recent careful examination of dental wear patterns indicate he is an herbivore.
Why does it take so long for a terrestrial vertebrate, a tetrapod, to get around to eating plants? Well, consider what a plant is made up of. As Karl said, it is largely cellulose. Do you digest cellulose? No, it pretty much passes through. You need to work on learning how to digest cellulose. You can do it by a number of mechanisms. You can invite a bunch of cellulose digesting bacteria to take up residence within your stomach and recover energy from cell contents and cell wall, or you can chew the stuff at great length to physically break it down. In this latter case you get what you can out of the cell contents and pass the cellulose of the cell walls on through. Either adaptation will require time to evolve. I think that one controlling feature is size. You probably have to reach a certain gut size before you can consume enough plant material to retain it long enough to get enough energy out of it to make it worth your while. If an herbivore has a specialization to high-energy foods, say to feeding off fruits and nuts, you can be small. But, if you are feeding on just general roughage in generous portions, you need to be fairly large in order to hold and digest the material. Hence Edaphosaurus has a small head and big body. Therefore, I suspect one of the limits is that we had to get to a point in vertebrate evolution where the tetrapods could be large enough to have a big gut, in addition to potentially setting up some sort of a relationship with bacteria in the gut.
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