Here's why:
* Unusual is an adjective describing something that is not common or ordinary.
* Uncommonness is the abstract noun that represents the state or quality of being unusual.
Other possible abstract nouns could be:
* Novelty: This emphasizes the new or unique aspect of something unusual.
* Oddity: This implies a strange or peculiar quality.
* Singularity: This highlights the distinctiveness of something unusual.
The best choice depends on the specific context and the nuance you want to convey.