Why is a Denver Omelet called omelet?

A Denver omelet is called an "omelet" because it is, in fact, an omelet.

The term "omelet" refers to a dish made of beaten eggs cooked in a pan, typically folded in half and often filled with various ingredients.

The "Denver" part of the name refers to the specific ingredients often included in this type of omelet: ham, onions, and green peppers. This combination of ingredients is often associated with Denver, Colorado, giving the omelet its name.

It's a bit like calling a "California burrito" a "burrito". The "California" part just identifies a specific type of burrito with specific ingredients.