Well, that's because Sayonara doesn't really translate properly to
"farewell." It's my understanding that people only say "sayonara" when
they never expect to see that person again. Otherwise, you're right,
"farewell" would have sufficed... under other circumstances.
Now, beyond the school-bench, my practical experience of Japanese
only comes from watching more anime than is healthy (and too many
volumes of manga, judging by the amount of squeezing required on my
bookshelf), but based on that, I would say that added sense of
finality in "sayonara" as compared to "farewell" is not enough to
warrant it's use here.
IME, sayonara can be and is used in less final partings (it does
imply that one expects considerable time to pass before roads next
cross).
YMMV; I won't try to pretend that I'm an expert. It's also possible
that I may be interpreting "farewell" (for example) too heavily,
English not being my native tongue, in which case I apologize.
---
Ronny Hedin, thark@mangakai.org, http://i99ronhe.island.liu.se/
"Baka baka, minna baka." -Hoshino Ruri, Nadesico
"Remember, there are no stupid questions, just stupid people."