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).
There were several threads on the josm-dev mailing list discussing the switch since about a year ago, and from what I recall, the reasons were along the lines of
some problems being more cleanly solveable with 1.6
1.5 causing problems with the build server infrastructure
few users (<4%) still using Java 1.5, with some of them probably able to upgrade to 1.6
no API update being expected for the near future, which means that old JOSM versions will continue to work
The threads I was able to locate despite the archives’ lack of adequate searchability are these: 123
what kind of problems are those? I don’t see mentioned anything palpable in the linked discussions
I didn’t look into your build infrastructure, but if you’d use Maven you shouldn’t have such problems.
4% is not a minor amount of users. Over years the worldwide market share of Mac OS was quite below that. Now it is at about 5% worldwide. The amount of Linux-Users worldwide is below 1% btw.: http://gs.statcounter.com/#os-ww-monthly-200908-201008
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.