- the number 77 is a smooth number, fits Sanji
- significantly higher than most first bounties, makes him look good but was late enough to fit "third place"
- hilarious drawing with a whole side story devoted to it
- establishes the theme of getting a good bounty, but having it also hurt his pride
No, Alabasta was not the time. It would have taken away a little from Zoro to give Sanji little in exchange.
For a reason in story, I think you can just point to not having enough presence. Luffy didn't get a bounty for his first few encounters.