Here's why:

* Evolution is about changes in allele frequencies: Evolution is driven by changes in the frequency of different versions of genes (alleles) within a population over time.

* Genes are the units of inheritance: Genes carry the information that determines traits, and these traits are the ones that are subject to natural selection.

* Mutations happen at the gene level: Mutations, the ultimate source of new genetic variations, occur at the level of DNA sequences within genes.

While evolution can be observed at higher levels (populations, species, etc.), the fundamental changes that drive it happen at the level of the gene.