What makes everyone consider Shulk a shoo-in? To my knowledge, we haven't seen any Xenoblade content in Smash yet nor can I think of any hints towards his inclusion. I'm not saying that he's unlikely or anything as I think he'll be playable, but I don't think he's as likely as many are assuming.
I'm going to give you the quick list:
1) His game was wildly popular among fans and critics, some calling it the "rebirth of JRPGs." Like... greater than Mother-tier love.
2) Unlike things such as Mother 3, fan demand and collaboration was able to get the game to have a western release which sold VERY well, despite limited copies being released due to poor predicting of western popularity.
3) Moveset potential, much beyond "just another swordsman." Visit the Shulk Thread for that.
4) One of the most desired characters from the Japanese polls.
5) Sakurai tweeting a picture of a model of the Monado, the main weapon that most of the story revolves around.
6) Monolith Soft retweeting some updates about Sm4sh (including the reveal trailer that ended up being Robin/Lucina, people believed it being Monado Monday)
7) The large PR amount for Xenoblade Chronicles X, meaning that Nintendo as a whole has big hopes for the series as it moves on.
8) The english VA for Shulk saying he was doing "big work" for this year but won't say until later in fear of "my balls being cut off," as he jokingly put it.
9) The overall track record of Gematsu which had some inside information at some point of time. Unlike Chrom or Chorus Kids, there is little to no competition for a character to represent the game/series that would be as big as Shulk.
I don't want to say a "shoo-in." However, I say Shulk is one of the most likely newcomers from those often discussed.