Originally Posted by MGhareeb
Nobody, in the same way that nobody came up with the "rule" that you should say "excuse me" when you bump into somebody while walking down the sidewalk.
It's basic, common courtesy and politeness. You've been given several links that explain the logic behind it, but let me put it this way: I tend to not help crossposters, simply because I have no idea what help they've received elsewhere. If you posted on one other forum, chances are you posted on about 15 other forums as well, and I have no idea whether you are still looking for an answer or have found one hours or days ago.
Taking that into consideration, crossposting also tells me that you don't really care whether I waste my time answering a question you already had answered at some other forum- which means that you think your time is more valuable than my time, or that your question is more important than the other questions being asked- which is extremely rude and disrespectful.
That being said, you should notice that I never said you broke any rules (of this forum anyway). I didn't make a big deal out of it, I simply pointed out the other answers you were receiving, so that other people wouldn't have to waste their time repeating answers you were receiving elsewhere.
So you can thank everybody else "except me" all you want. But part of your goal should be making people want
to answer your questions, shouldn't it?