But I am in Computer Science - I know that the situation is not at all the same in other disciplines.
I think if you want to be cited, it is kind of silly to publish at a venue where your paper is not immediately accessible - there's usually enough other papers to cite, so it's in your own interest to be available.
That said, regarding the original question ("why would anyone not do this"), I know from previous discussions here on HN that I'm part of a small minority, but I wouldn't do it because I find it unethical. There's lots of things that I wouldn't mind having / owning but I cannot afford them. Just because they might be available through non-official channels for free does not mean that using that option is the right thing to do.
In this line of thought, I also think it is irrelevant whether putting publicly funded research results behind a paywall is ethical on the publisher's side: two unethical acts don't make an ethical one.