Because people still love it, even if it is seen as "old" now.
If people struggle with a certain aspect of the game, or just want to trade with others, what's wrong with doing that?
Yahoo Answers is here for us to find answers, so surely there's nothing wrong with asking questions. Who cares if the majority of things are about Pokemon?
Pokemon was and still is a huge thing, and it shouldn't matter what age you are, you can play what you want. Also, some people may not want to use a walkthrough as they are usually boring, and it takes you forever to find the bit you are upto as Pokemon games are so long and complex at times. Also they are not always reliable, so why not just ask some people who will answer questions for you anyway?