Because under the Unicode decomposition rules, ø is not decomposable. I can't explain why that is the case (probably because there is no reason to have a combining /. After all, the only languages that use ø are languages that use it as a character of its own).

On a related note, I would expect a search for ö to match ø. As would you, I guess?

In the thread on the Unicode mailing list, the recommendation seems to be to use the CLDR (http://cldr.unicode.org/). Of course, this assumes there is a locale, but the choice of locale can easily be customisable (with the default being the user's locale).

Another poster on the same thread mentioned that the CLDR doesn't go all the way, but adding a set of exceptions on top of it shouldn't be hard. In any case, the result would be significantly better than what is implemented now.

Regards,

Elias