Using low tiers at a competitive level is admirable, but you still haven't given me a good reason why one shouldn't use a character simply because they're good. If money is on the line, why NOT use the best characters?
People generally don't use low tiers even at a highly competitive level for a reason: they are boring. Low tiers like Melee Kirby simply can't accomplish the amazing feats that Marth and Falco can. They are boring to use and there isn't much to learn about then.
And please don't say that anyone can master Marth. It makes you sound like a n00b. Effective spacing is harder to use than it seems, especially at a highly competitive level. Go ask Azen, Ken, or M2K if Marth is an easy character to use.