The reason is because the color orange was named after the fruit; the fruit wasn’t named for the color!
Also, fun fact: It used to be “norange,” but the n migrated. So it changed from “a norange” to “an orange.” (You can see it more clearly when looking at the Spanish word for orange, naranja.)
This also happened with the word “uncle.” It used to be nuncle. “a nuncle” —> “an uncle” (This is why George R.R. Martin uses “nuncle” in A Song of Fire and Ice… it’s the older form.)
The phenomenon is called a juncture loss. :)