Because melee is mostly figured out, there is very little theory crafting that still needs to be done. That leaves the discussions to mostly lower level players who are trying to catch up. There is very little incentive for someone like Mango, M2K, Armada, etc to spend their time browsing the boards correcting people for their mistakes or giving out advice. From their perspective that would be doubly wasteful because 1) it exposes their tactics, which opens them up to counters, 2) their time is better spent playing the game or doing other things (work, studying, etc) and 3) 99% of the questions asked have been asked and answered time and time again. It doesn't take a top10 player to answer these; in fact a simple google search might be in order.
If you really want to see discussions with these top players, go back and read the hundreds of pages in the character specific "ask ___ questions" threads. You will find tons of content from people like KirbyKaze, Dr. PeePee, Armada, Leffen, Fly Amanita, Wobbles, MacD, Jman, SilentWolf and even Magus himself!