They seem to be marketing it to Deadpool fans moreso than anyone else. They are likely thinking that there is enough of a cult following to get a word of mouth campaign going (guys showing their friends trailers, dragging their girlfriends to it, etc..). It's budget is pretty modest for a superhero flick, so they likely don't want to risk a huge marketing campaign especially when there's only a certain niche of people who will be seeing this anyways.
They're basically using their built in audience to market it for them. To be honest, it's working. I didn't know much about Deadpool until this initial test footage stuff, and I'm a big comic guy. It's the people that love him and are getting super jacked on the idea of a movie that got me interested in seeing it. And now I have buddies who literally had no clue who/what Deadpool was, and still don't, but they want to see the movie.
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