I came upon Marth by accident in Melee. I never really had a main in the original, though Fox, Samus, and Link were the closest. In Melee, I liked Link, and so when I saw another swordsman, I used him too. No idea who he was, but he was fun. When Roy showed up, I used him too, but I had more success with Marth. He and Link were two of my most-used, along with Zelda. I'm a Zelda-phile (the games), so I felt compelled to use the characters. Zelda clicked for me, since I became decently beastly at landing lightning kicks.
In Brawl, Link was painful to recover with, and my old staple, the spin attack, was less than useless. So Marth and Zelda are now my 1 and 2, purely by luck and just falling into them, rather than me ever actually choosing.