Because it has an impact on the community as a whole, if you have the ability to not view submissions, you dont get to downmod them if they arent "in the spirit"
Look at loosely computer related obama articles, or techcrunch ones, they are inevitably posted and quite a lot of people dont like to see them. Given the ability to block them means they would receive less downvotes and make articles that a lot of people dont want to read, more likely to go on the frontpage. combine that with the impression of new users (who havent blocked anything) and its a cycle.
"Given the ability to block them means they would receive less downvotes and make articles that a lot of people dont want to read"
Errr ... downvote comments I understand, but downvote articles? It's unpossible! Personally, I'd like to be able to hide or filter articles I'm not interested in. User blocking, I would agree, is antisocial and not in the spirit of things.
You can't downmod submissions. And comment downmods already act as a filtering device by graying out the text of unpopular comments. I doubt many HN regulars would say that the site today suffers from a lack of comment downmods. Those who would choose these hypothetical filtering options would be choosing to passively consume HN as readers rather than as participants. Those who wished to remain active participants would be able to do so. This happens already, where a minority of "leaders" drive most participation and a silent majority consumes the site passively. The only difference would be that you'd be giving tools to the silent majority to make their experience more compelling by automating the filtering that they already do manually.
Look at loosely computer related obama articles, or techcrunch ones, they are inevitably posted and quite a lot of people dont like to see them. Given the ability to block them means they would receive less downvotes and make articles that a lot of people dont want to read, more likely to go on the frontpage. combine that with the impression of new users (who havent blocked anything) and its a cycle.