Marth was in because hes the best Fire Emblem representative for SSB. He was the #11 requested newcomer for Melee in Japan, which was above the likes of Meta Knight and Diddy Kong, and only below the biggest names like Bowser, Ganondorf, and Peach. Hes the most iconic Lord of the series, a favorite in Japan, and has a game coming out in the future that is a remake of his first game, Fire Emblem DS, which will presumably bring him international. Hes in because he deserves it, simple as that.
Melee stages were included because people liked them. Do you have any idea how many people were asking for the return of Hyrule Temple? Hell, after we learned what stages were in, there was some significant complaints on the stage board about which ones didn't make it.
Sheik I don't know what to say about, since shes an oddball character I can't really see any objective reason to include or any objective reason to exclude.
Zevox