I have no clue why they decided to go with Alvida as the first episode rather than the flashback as seen in the manga. The flashback is the perfect set up as it introduces you to Luffy, Shanks, the hat, and his goal all in one episode and it created a bond with Luffy from the get go. That way, when he fights Alvida the next chapter, you're rooting for him to see how far he's grown since being a little kid. By making Alvida the first episode, there's little to no attachment, single-handedly making it a boring first episode.