Here's why:
* Literal meaning: A fence is a physical barrier.
* Figurative meaning: "On the fence" refers to being undecided or neutral about something.
* Comparison: The phrase compares the state of being undecided to being physically positioned on a fence, unable to commit to one side or the other.
This figurative use of language creates a vivid image and helps convey the feeling of being indecisive.