Why? Because otherwise, you will be wasting someone's time. Everyone who answers questions in the BigMooseSaloon is a volunteer, donating a precious resource: their time. If you post the same question to two forums, and two different people each post essentially the same answer to one of the two threads, then one of those two people has wasted her time. That's a shame -- there's only so much time to go around, and because of that waste, someone else's question might not get answered.
Worse, if one of those two people finds that other copy of your thread, how do you think they'll feel about you and about JavaRanch? They'll be a little angry with you, and they won't be so quick to volunteer their time at the Ranch in the future. If that happens a lot, then no one will be left to answer questions!
Choose it carefully
Since you're only going to post in one forum, give it a little thought. If you just started learning Java last week, don't post your question in Java In General, even if you think it's a hard question. If you have a question about setting up Tomcat, don't post it in the JSP forum, even if that's the forum you read the most.
You shouldn't try to post your question in the forum with the most traffic; post your question in the forum where it will be most on-topic, because that's the place where it will get the best answers, the fastest. If you post your Threads question in Java In General just because that latter forum gets more traffic, you'll find that a bartender will quickly move it to the Threads and Synchronization forum, anyway. This will waste their time, which has various bad consequences, as discussed above.
If all the questions in each forum concentrate on a specific topic, then it's very easy for people to concentrate on supplying useful answers. If most questions are on-topic but some are off-topic, then those off-topic ones are likely to be ignored. If all the questions in a forum are unrelated, random posts, then that forum becomes so unpleasant to read that people will avoid it. Please help keep the BigMooseSaloon neat and tidy -- choose your forums carefully!
Taking the time to find the best forum for your question demonstrates that you care enough about your question and everyone else's time enough to ShowSomeEffort when presenting it.