Because:
and Bostrom's impact from publishing preprints directly seems to have been larger than people who happen to subscribe to those particular journals.
I'm not an expert on philosophy but at least in my own field (math) people are more likely too look at a preprint that is accepted in a major journal than look at a preprint otherwise. This isn't a strong effect but it is definitely present.
Recently, I gave some reasons for SIAI to begin publishing in mainstream journals, and outlined how it could be done.
I've recently been made aware of some pretty good reasons for SIAI to not publish in mainstream journals, so here they are: