This is simply because long comments very rarely provide more value than short ones. In fact, I would wager that shortness is a good quality that shows the poster tried to condense his point into its true essence.
Example: The parent post could be ~ 5 lines and contain the same point. I really like the way 4chan handles this sort of thing: The so called "green text". On 4chan it is unspoken rule that nobody cares about you. In order to make your text interesting enough to be read, you have to make it as short as possible while still carrying your point across.
The inherent problem with this is that certain posts simply cannot be made shorter without omitting useful information. When users are expected to write short comments, you'll quickly find that one-liner jokes and the like are the most upvoted, not the ones with actual content.
While 4chan may be a fun way to pass time, it's hardly the place I go when I want to read/join intelligent discussions or learn new things.
The degeneration of HN is likely due to the influx of new users from 4chan and Reddit, where comments are very short and often appeal to the lowest common denominator.
[S]hortness is a good quality that shows the poster tried to condense his point into its true essence.
This is often true, but I think the value of brevity comes mostly from the context, a threaded comment page. In that context, users expect a discussion, not a monologue. They're not just looking to passively absorb information by reading, they're looking to sharpen their own understanding by actively articulating a response. That's harder to do when you're trying to respond to a comment that makes multiple points. Better to break a long-form comment into several more tightly focused short comments so that people can more quickly pick out the topics they want to respond to.
Example: The parent post could be ~ 5 lines and contain the same point. I really like the way 4chan handles this sort of thing: The so called "green text". On 4chan it is unspoken rule that nobody cares about you. In order to make your text interesting enough to be read, you have to make it as short as possible while still carrying your point across.