Looking at our own planet, I would guess that it's a combination of a) it being simply infeasible to send large animals to distant stars (we haven't yet colonised the moon or even made a serious attempt to colonise low earth orbit, and we might not do so before we run out of cheap energy) and b) technological singularities.
I don't know. Robin Hanson blogs a lot about it, but none of the steps seem terribly plausible although my favorite currently is big brains being feasible - in a number of ways, heads seem to be rare, big brains even rarer, and the costs of big brain almost too expensive to bear because neurons seem to be unable to get more efficient; one paper I liked on the topic was http://www.pnas.org/content/early/2012/06/19/1201895109.full... but you can find a lot of relevant material in http://www.gwern.net/Drug%20heuristics
I imagine that if there were another planet in the solar system capable of supporting native life with just 18th-century technology, we would already have sent people there, even if it were a multi-year trip. Which would provide big economic drivers for interplanetary transport.
Although I'd immediately agree that those are filters hindering the spread of any humanlike civilisation, it doesn't at all stop the spread of other, technological forms - the "vile offspring", to borrow a term.
Honestly, I have no idea what might work as a future filter, at this point. I don't think the expense could be it; that's going to drop, quite abruptly once uploading (or straight-up AI) works well enough.