The main reason Pit was included was most likely from fan pressure. The guys at IGN have bothered Miyamoto about it for years (and I love them for that!), and included a kid icarus game in every wishlist or anywhere they could. The last time they asked, at the last E3 Prior the launch of the Wii, Miyamoto seemed to agree to bring him in some form.
IGN: "OK wtf are you waiting for to bring kid icarus back, dammit"
Miyamoto: "OMG noone knows that little bastard, noones gonna buy that game"
IGN: "Hell yeah they would!"
Miyamoto: "srsly?"
IGN: "YEAH"
Miyamoto: "Would you buy a game with pit in it?"
IGN: "I'd give my virginity for it"
Miyamoto: "OHOK I'll see what I can do"
Not the exact words, but that's how I remember the interview went on. One year later they revealed the wii, and later on, the trailer with pit on it. I jumped in joy. I bet the IGN folks did too.