It's much less efficient for both YC and individuals if every interested individual started emailing and asking for updates. Much better this way - after all that's why broadcast or pub-sub systems exist. HN is kind of a curated broadcast system.
It often happens that people who're working on the project might not find 'right now' to be the ideal time to share updates. This could be either because they're immersed thinking through a particularly challenging aspect of the problem/solution, or they feel a little lost (which is a normal feeling in the middle of ambitious projects). Sharing an update at that point makes you feel vulnerable, but that's exactly what you need to do.
Or it could be that they have a really promising angle and want to see more data before writing something up. The 'more data' approach is usually a mirage (promising approaches start showing potential even at early stages/small scale) and sharing progress helps the project even in that case. (For example, you might learn that a similar approach has been tried before, and here is what someone learnt).