I'm guessing they just wanted something different and new (to D&D anyway) when they chose to use Rokugan for Oriental Adventures instead of Kara-Tur. Likely the folks who were going to work on the project were really into Legend of the Five Rings and wanted to use its setting. Or maybe they figured that they'd sell more copies of OA if it were being supported by AEG's supplements, since WotC didn't plan to support it themselves.
If you want OA for the setting/flavor, just buy AEG's Rokugan book instead. It's much more focused on the setting of L5R, and reprints/replaces several bits of rules material from OA anyhow, whereas OA is more a grab-bag of assorted stuff for various kinds of oriental setting.
OA still has some useful and interesting stuff when choosing not to use the Rokugan setting. Several good prestige classes (blade dancer, ninja spy, singh rager, tattooed monk, witch hunter, etc.), some neat and useful magic items (shaman's bones armor, flying phoenix sword, nekode of spider climbing, courtier's obi, eight diagram coins, flute of the snake, porcelain mask, wilding clasp, etc.), the Iaijutsu Focus skill, a handful of neat martial arts feats, martial arts mastery abilities, a good Monsters chapter, and some Taint-rules-related stuff not in 3.5 books (to my knowledge; the basic Taint rules are reprinted in Unearthed Arcana I think, and a variant of them is in Heroes of Horror IIRC, but the extra bits like maho-bujin and akutenshi aren't, I think).
That said, it obviously isn't much use if you don't plan on using any oriental region of your chosen campaign setting very often. Very little in Oriental Adventures is useful outside of oriental campaign regions, though you could certainly use the monsters and a few prestige classes elsewhere (giant toads and yetis are entirely appropriate elsewhere, as are the bear warrior, shadow scout, and witch hunter, as well as possibly the eunuch warlock and singh rager; other monsters and PrCs may be a bit less likely to fit well).
Wizards of the Coast isn't gonna make a Revised Oriental Adventures. And can you imagine if they did? Just look at how they mangled and munchkinized the Psionics Handbook into the Expanded Psionics Handbook and Complete Psionic...... The 3.0 PsiHB only had one or two broken powers far as I know, and only if the DM wasn't using the assumed average nuumber of encounters per day and such. A little tweaking, weakening, and expanding would have made a fine Revised PsiHB, but nooo, what we got was the Expanded And Unprecedented Crystalpunk Munchkin Psionics Handbook.
Wizards is going to keep making their environment-based books, their 'races of' books, their 'complete' books (what a horribly false name for them, too), their 'heroes of' books, and possible additions to the 'tome of' and 'codex' books (likely a Fiendish Codex II and maybe III, maybe some Planar Codex books or Celestial Codex books (although quite unlikely), a Tome of Magic II perhaps, possibly a Tome of Battle II, probably a Tome of Adventure or somesuch, etc.). And of course, a few more Eberron and Forgotten Realms supplements. If we're lucky, Wizards will finally make another Greyhawk book or some manner of real Planescape support (not the meager scraps they toss in Manual of the Planes, Planar Handbook, etc.). It's doubtful that they'll provide any Oriental Adventures support, or any more republished settings (besides that Ravenloft book they're making, IIRC, but then, Ravenloft was already supported for a while by another publisher with 3E). Further psionics support is doubtful, though there may be a few scraps for it in some of the last few 3.5E books put out before Wizards decides 3E is too bloated and wonky and stagnant, moving in to cash in on 4th Edition (more like a 3.95 edition I'd bet, but we can always hope they'll put more effort into it and be innovative again like they were for 3.0's release).