Risk aversion seems to be the killer in terms of timeframe. Burt Rutan famously remarked a few years ago that if we're not killing people, we're not pushing hard enough; the test pilots that made up the early core of NASA astronauts knew it was high-risk and were okay with it, which is why we got to the moon in less than a decade (and yes, people died along the way). Modern-era NASA is just so terrified of bad PR that they won't decide on a strategy for tying their own shoes without spending a decade ensuring that nobody will inadvertently strangle themselves with the laces.
It makes sense for NASA to avoid anything that would significantly decrease the public's opinion of them (as that would eventually lead to funding cuts). But I don't think their assumption that the death of astronauts/pilots would negatively affect their image.
Lets say Curiosity crashed into mars... that would be a HUGE problem for NASA (3 billion dollars wasted, people would say). But if a couple of astronauts died on the way to Mars... I think that would only strengthen the resolve of the American public. No politician would go on record saying their life was wasted. Instead, their death would be framed as heroic (rightfully so), and I think there would be a push to try again, and to try harder.
Given that logic, what NASA views as risk mitigation (send a robot to mars instead of a person, so we can get better at it before risking lives), is really is a very risky strategy from their organization's perspective.
There is probably data out there that backs/disproves my general logic here (sentiment of nasa after astronaut's deaths, vs after loss of robots). I'd love to see it.