Why do you think it's like this? Are those left expressing the nihilism of the rejected? Surely, the successful (the kind of success that YCombinator strives for) are eternally optimistic.
It's easy to spot in comments, posts, and articles–they have titles like:
"... is actually bad"
"... is actually good"
"... considered harmful"
"Actually, ... "
"No, you don't ... "
(1) The "senior" members do not set a good example, to say the least. People who have spent ten years honing their rhetorical skill and accumulating karma/fans so that their comments float to the top of every thread have a disproportionate influence on the overall tone. Too many of those seem to think their own startup idea from ten years ago was the absolute bees' knees, therefore anything else must be lame by comparison, and they're eager to say so at every opportunity.
(2) A lot of newer members have grown up (professionally) at FAANGs that are very opinionated in their own way, which is quite contrary to how startups work, and they are accordingly skeptical of anything startup-related.
Put those together and a torrent of negativity is pretty much inevitable.