/u/LiveLaughCry makes some good points, but I disagree with a lot of their points so I'll write my own response.

The first point is that, I'm actually not sure most reptiles are carnivorous. I can think of several reptiles that are herbivores or omnivores but it may be possible you actually are correct; I doubt anyone has an intimate-enough knowledge of all reptile diets to verify it. In fact, we don't know a whole lot about what anything eats -- food webs, or data describing the networks of who-eats-whom in an ecosystem (my field) are scant and poorly assembled with rare exception.

We know even less about what influences diet over evolutionary timescales. Energy budgets like the scenario described by /u/LiveLaughCry have equivocal empirical support. A huge body of literature suggests that diet is influenced by many things -- namely, body size and the nebulous concept of "niche space." A small body of emerging evidence also hints that over evolutionary time, omnivory may emerge as top predators in certain types of habitats are tempted to "short-circuit" the food chain and feed from the bottom. This is a really new idea and I'm not sure about it yet, as it gets a little close to community selection, albeit non-adaptive selection.

What's really cool is that mathematical models suggest diet is strongly influenced by body size and the dimensionality of the habitat you forage in, through effects on encounter rates with resources -- you are more likely to encounter/perceive resources with a certain certain fraction of your body size depending on whether you are searching in 2D (e.g., a large mammal foraging on a grassland) or 3D (e.g., a carnivorous fish foraging in the pelagic zone). This suggests a feedback between diet which influences body size which in turn influences diet (typically over evolutionary time, although their are examples of such shifts within a species over ecological timescales -- ontogenetic diet shifts).

The point of that spiel is to make this point: even if we knew most lizards were carnivorous, answering the "why" part is really hard right now, as we're only beginning to learn about how interactions between species emerge and how they influence ecosystem dynamics.