Here's why:

* Mendelian traits follow simple patterns of inheritance, typically governed by a single gene with two alleles (versions of the gene). These patterns were first described by Gregor Mendel, hence the name. Examples include eye color, widow's peak, and some genetic diseases.

* Non-Mendelian traits, however, exhibit more complex inheritance patterns. They can be influenced by:

* Multiple genes: Several genes can contribute to the expression of the trait, with each gene having a small effect. This is called polygenic inheritance.

* Environmental factors: The environment can play a role in shaping the expression of the trait.

* Epistasis: One gene can mask or modify the expression of another gene.

* Mitochondrial inheritance: Some traits are passed down through the maternal line, through the mitochondria.

Examples of non-Mendelian traits:

* Height: Height is influenced by multiple genes, each contributing a small effect, as well as environmental factors like nutrition.

* Skin color: Skin color is determined by multiple genes involved in melanin production.

* Diabetes: While there are some single-gene forms of diabetes, many cases are caused by a complex interplay of genes and environmental factors.

Therefore, non-Mendelian traits deviate from the simple patterns of inheritance seen in Mendelian traits, and are not controlled by a single gene.