They did originally try the "Rah Singularity" strategy. Only more recently did they switch to using more negative marketing.
See Singularity University and the Singularity Summit. Cheerleading is more effective in raising money (and there are many more things to do in that line), but money to cheerlead and accelerate, not to try to shape outcomes.
Many people complain that the Singularity Institute's "Big Scary Idea" (AGI leads to catastrophe by default) has not been argued for with the clarity of, say, Chalmers' argument for the singularity. The idea would be to make explicit what the premise-and-inference structure of the argument is, and then argue about the strength of those premises and inferences.
Here is one way you could construe one version of the argument for the Singularity Institute's "Big Scary Idea":
My questions are: