Many “obvious” features are missing from the JDK and its various APIs. Why this particular feature was omitted / forgotten? We can only guess. But your question has been a RFE at Sun/Oracle for a long time:
- https://bugs.java.com/bugdatabase/view_bug?bug_id=6278287
- http://www.java.net/node/644910
You could support those RFEs by commenting on them. Note, here’s an authoritative answer by Joshua Bloch on the subject:
I vaguely recall considering it, but I can’t recall whether we
explicitly rejected it with good reason. We were running very low on
time when I implemented EnumSet and EnumMap, and it’s possible that
time played a role in our decision
http://comments.gmane.org/gmane.comp.java.jsr.166-concurrency/2158
So even he had to guess 🙂