I don’t know what the reason for this is (I hope it’s not just the urge to be “modern”), from my understanding the advances of Java 6 over Java 5 are few (and I am a professional Java Programmer, e.g. I earn my money with this). On the other hand this move alienates all the folks who can’t upgrade to Java 6, Mac OS X users on PowerPC for instance (Java 6 for Mac OS X is available only for the x86 / Mac OS X > 10.5 versions of Mac OS X). That’s a pity since my private main machine is still a PowerBook G4 (for reasons I won’t discuss here).
If you want to reach the developers of JOSM with your comments, then you should mail the list you read the discussion on. They are not likely to reply here, because they are probably not even reading this.