Because the writers and director of "A New Hope" wanted the ship to sound both futuristic (At the time of Star Wars' release in 1977, the new Millenium was almost 23 years away), as well as faaaaaast (The Falcon is a predatory bird, and its primary means to capturing small prey like other birds/mice/fish/Calista Flockhart(*)/etc is to locate and observe them stealthily while gliding at great height, then descend on them rapidly like a gravity-powered heat seeking missile).
Given the visual design of the ship, which as a necessary plot point looks both old/decrepit and sluggish, the juxtaposition with the name also serves as occasional comic relief throughout the course of the film franchise.
(* Now, doesn't that relationship make a lot more sense?)
One thought on “Deano’s answer to: “Why is it called the Millennium Falcon?””