It seems people have used how often a character wins a tourney as a valid defense for not playing low tier. But personally, I think many people who ignore the tier list are also people who play more casually, which to me means that the reason there are more tourney players who win with fox or marth is simply because there are more competitive players who actually PLAY as those characters. An why are there more of those? Because there are definetly people who have read about how much Ganondorf, Mewtwo, Link etc. apparently suck according to some, and how good Fox, Marth, and Shiek are, and have accepted that without actually pausing to question it or see if it's true through a play test.
It is true that lower tier have more limitations. But the fact that other higher tier characters have more natural advantages doesn't mean **** if those advantages don't do anything against the apparently "inferior" character. Sure, it most likely means that the lower tier character can't do anything either, but at least their not losing or getting damaged.
And also one more thing, we ARE ONLY HUMAN. Which means it's absolutely impossible for us to fight a "perfect" match when two players are fairly even in skill level. And it also means that we may make costly errors in our performances that give lower tier characters a victory. And keep in mind that some characters are harder to use to "maximum" effect than some lower tier characters. And If the two get into a complete stalemate that stretches on for a long long while, dollars to doughnuts that the player playing the higher tier character will screw up simply because of their characters bigger demands for concentration. And if a character is hard to use, it also means they fall into similar routines no matter whom they might be facing. In that case a low tier player doesn't have to use a "better" character, they just need to correctly predict when to dodge and hit.
Finally, there are also many emotional skills that come with everything. Don't you think you'd need an enormous emount of dedication to not think about personal problems, how tired your eyes are getting, how bored and frustrated your getting with the stalemate, how to fight through your aches in your fingers, how to not give in to pressure, how not to get too excited, and just how important this is to you anyways? Well those are just my thoughts on this question, I hope I've been able to help.