Here's why:
* Verbs describe actions or states of being: "Falling" describes the action of moving downwards.
* Nouns name people, places, things, or ideas: You can't point to a "falling" as a physical object.
Example:
* The leaves are falling from the trees. (Here, "falling" is the verb describing what the leaves are doing.)
To make "falling" into a noun, you could use a gerund:
* The falling of the leaves was beautiful. (Here, "falling" is a gerund, a verb form acting as a noun, referring to the act of falling.)