The answer that jumps out to me is that Java has been around since 1996, and C# is relatively new, and so is SO, so you can imagine most Java developers have been getting their answers from somewhere else, and they are sticking with those sites. These sites, unlike SO, are (or at least originally), have been specifically devoted to Java, and have long preceded SO so are very established and have their core set of users:
Here are a few to check out (I'm a newbie on SO so you'll have to craft your own links. This is getting ridiculous, these restrictions are bordering on annoying, I'm starting to like SO less and less. We'll see.):
First, it seems you are interested in Struts. It is always best to find a list that the actual developers follow regularly. Want an answer by Craig McClanahan himself (maybe) or Ted Husted? Try the site below:
Java Ranch must be one of the grandaddies:
Java Lobby is a great site, very interesting discussions, and you get some of the top names in the biz posting questions and answers.
The comp lang * have been around forever, since the beginnings of the modern internet. Seem to be having some spam issues. I know, like Java Lobby though, that some of the top minds follow these, or use to, especially in the C, C++ and CORBA world for example (those are the ones I am familiar with)
Sun Java Forums (now Oracle of course). This is a good one, you'll find yourself here (well, somewhere on Oracle) especially if your question is related to a bug because these guys will be the ones working on it.
IBM DeveloperWorks has some good content, excellent articles by some of the IBM experts, but probably less traffic than some of the above.
JavaWorld is an excellent source, not really a forum, but has a tremendous set of articles. At one point they were going to shut it down, but thankfully they somehow got funding so it is still out there cranking away.
That said, from the posts here, it seems there are a fair number of Java questions and answers here, so the other possibility is the standard answer at any site - you need to work on your questions. Either they are too general, so that people can't even be bothered to say "google it", or they really are tough ones that nobody has an idea on, or they just plain aren't interesting, or no one has really run into your issue. Start with reviewing the link below for proper etiquette (or even read the Stackoverflow version, any any other):
And that answer is regardless of whether we are talking about SO, or Java Ranch, or Javalobby, or the Sun (Oracle) forum, the comp.lang.java, or the myriad of other excellent resources.
I'd suggest trying your questions on the sites above, see what kind of response you get. Give as much detail as possible, and show you've tried to solve the issue. What version of Java are you using? What OS? What other libraries? IDE? Did you give the stack trace? Give a decent idea of what you are trying to do, but not overwhelm the reader with a huge file, i.e here's my jar file, it doesn't compile, why doesn't it work?
You'll gradually follow one more than the other, but note that if you go through Google, in theory google is unbiased and will pull up answers across all these sites. Which gives me an idea of some sort of smart aggregator - maybe Google is working on this. The ultimate answer site.