Why did Sun/Oracle choose Unicode 5.1 over Unicode 5.2 in Java 7/JDK7?

Answer 1

For a definitive answer, you'd need to ask Sun/Oracle through formal channels, and I expect that the answer would be something like "the feature list for JDK 7 has not been finalized".

Wait until it JDK 7 is finally released ... it might change between now and then.

Right now, it could just be that the project team member who would dealing with this is currently working on something else.

EDIT

Note that JDK 7's Unicode 5.1 upgrade was done in the M3 milestone. "M3 2009/04/03 – 2009/05/14 (b59)". Unicode 5.2 was released 5-6 months later in 2009/10, according to your question.

Answer 2

There's more behind the change according to the official pages:

6,648 new character assignments were made to the Unicode Standard, Version 5.2.0 (over and above what was in Unicode 5.1.0). The character repertoire corresponds to ISO/IEC 10646:2003 plus Amendments 1 through 6.

(More changes are on the page referenced above)