You are correct that sugarcane, beets, and corn have not been specifically excluded like Stevia and Monk Fruit. They may however, score lower in the novelty judging criteria than other entries.
While the goal is indeed to "find a "plant-based natural sweetener that has few or no calories", the challenge is is non-scientific in nature. Given this, it's likely that a person sharing a story may not know the specific calorie content of the plant they are describing, which is why it's not included in the judging criteria.
The submission form does ask about the cost of the plant in context of how it is normally obtained (if bought), but this is not a specific factor in the judging criteria and won't alter scoring.
The Guidelines page states in many places that the sweetener should be "derived from plants other than Stevia or Lo Han Guo (Monk Fruit)" and these are the only two excluded under the Novelty criteria. What about sweeteners derived from sugarcane, beets, or corn?
Also, while the challenge is trying to find a "plant-based natural sweetener that has few or no calories," the guidelines make no mention of caloric content and that is apparently not a judging criteria. Is that accurate, or are the submissions going to be judged against undisclosed criteria like these?