Because of the Steem API. Steemit is in a unique position to allow users to create their own trending tabs and moderate the comments on their blog. This same logic could be used to filter the feed as well.

Secondary concerns are instant signups. Simply make new accounts have 0 SP and you kill all the bots trying to exploit the system while letting anyone make an account instantly. Of course, there would have to be a function to delegate bandwidth (separate from SP) so new accounts could comment and post.

I'd also like to see a favorites list for users and individual posts.