I think Fiske was making a point against Enlightenment thinking which championed rationality over gut instinct but failed to make good the world’s injustices – or merely replaced some with other new ones, he might say.
It’s said – including by Fenster – that secrecy on the part of the authorities does promote conspiracism. But the recent open government approaches seem to have confused massive amounts of data with transparency, so it seems that openness on its own isn’t sufficient to make conspiracism go away. And in any case, as you imply, secrets are sometimes a good idea. If you’re conspiracy-minded, I’d speculate, surely the existence of any secrets whatsoever is a licence to give your imagination free range.
Yes, it’s an “attempt by humans to explain/accept that which they don’t understand and/or don’t know.” But an important thing about conspiracy beliefs is that they don’t tend to be held by all humans. Two good presentations from a psychological perspective are Chris French and Robert Brotherton (together) and Karen Douglas.
http://stephenlaw.blogspot.co.uk/2012/07/chris-french-and-robert-brotherton.html
Entirely separately from that, I tend to think of conspiracism as a kind of upward scapegoating.