For me:

1) The funds I have I might need later, and I'm not willing to take chances on my future. 2) Uncertainty as to whether the money would do much good. 3) Selfishness; my own well-being feels more important than a cause that might save everyone . 4) Potential benefits from SIAI are far and there's no societal pressure for donating money 5) I like money. Giving money hurts. It's supposed to go in my direction dammit.

I do anticipate that if I were to attain a sufficient amount of money (i.e. enough to feel that I don't need to worry about money ever again) I would donate, though probably not exclusively to SIAI.

Edit: Looking at those five points, an idea formulates. What if there was a post exclusively for people to comment and say that they donated (possibly specifying the amount), and receiving karma in return? Strange as it is, karma might be more motivating (near) for some people than the prospect of saving humanity (far!). Just like the promise of extra harry potter fan-fiction.