Everything is targeted by ficcers, slash and otherwise. If you mean what causes a work to get a lot of fic written about it, I seem to find that it's a function of how many characters there are (greater odds that any given fan of the story will find some pairing appealing), how popular the work is (ficcers feed off each other - feedback, beta readers, "challenges", community websites on which to post their fics), how well-developed the world's setting is (the more little, underused details, the more gaps there are for ficcers to fill; a setting that uses every element it has to the fullest has less "left to do"), and how much of a closed loop the plot is (I'm sure someone could write fanfiction on 1984, but it would not be a widely appealing challenge; it's pretty self-contained).