[wsxcde]

Consider the demographic submitting to NIPS. It's a self-selected group within the top researchers in the world in that area. The best people in the field don't want to be seen publishing the so called "second-tier" conferences, so they will submit exclusively to the likes of NIPS. And if you're an up and coming researcher or research group, you will want to establish credibility by publishing in these sorts of venues, and you will almost surely send your best work there. Add to this the fact that this is a "hot" field, so more and more researchers and research groups are getting into the field and trying to publish papers, I think it's very likely that NIPS gets a lot more good papers than they can possibly accept.