Eyy, wasn't around yesterday...
I tend to main characters when I either like the character or like their play style. Shulk is the latter, though he's grown on me as a character now. I haven't played Xenoblade, but I like his personality as seen in Smash. The versatility of his Arts is a huge selling point, and the range he has is ridiculous. If you can hit Marth/Lucina/Ike/any character with a counter with a tipped N-air or F-smash without taking the counter strike, that's golden.
Robin was my first Sm4sh main for both reasons: Robin is a huge favorite of mine (as is any FE character in Smash), and the fact that he was the first FE representative to use tomes in smash had me sold. I don't use him as much lately, since I honestly feel Shulk has better chances overall, but he's still a go-to for me with some MUs.
Link, I even used him in Melee because lol, I knew nothing of competition and swords are so cool, yay. <-- logic of a ten year old. Nowadays, I main as Marth in Project M, for similar reasons of strong air game, graceful battle style, and FE characters ftw.
I probably would gravitate toward one of the current "top-tier" characters, except for the fact that they are top tier. I can't stand following the crowd in competitive video games, and that's why I'll never be a national-level player. Who cares?
(Interesting fact though, of all the 3DS characters, only Fox was too difficult to clear All-Star on Hard with. In light of that, I viewed him as my weakest character... And since I know a friend who is not very strong in Smash but still likes to play, I gave him a handicap and would only play Fox against him for a while. And you know what? Now I'm good enough with him that I think I COULD feasibly main him... But won't.)