That's an interesting (and valid, imo) point. My guess is that on Twitter the prominent experts are less interesting because:
1. They're already speaking from a position of superiority in a certain sense, whereas on HN you're just in a conversation with everybody. You might be pretty important, but then maybe Alan Kay will jump in or something.
2. There isn't the same kind of prompting on Twitter that we get from the stories here which commenters are replying to. Instead, it's just the expert talking about whatever they want: maybe interesting to them, but not so much to everybody else.
3. The Twitter format doesn't encourage the same depth of discussion as HN, stemming from the initial character limit, but perhaps now just a vestigial consequence.
It could be interesting to have a site somewhere between HN and Twitter though—something that filters out more newbs/trolls, but doesn't exclude capable unknowns. Maybe you need an invite from someone involved in order to join, or maybe there are some questions you'd be required to answer which might establish you're on the right side of some threshold of intelligence, creativity, decentness, ability to communicate, etc.
Then again, I also agree HN is doing a pretty good job of ensuring good comments become visible.