Basically, reddit already exists, and an appropriate selection of subreddits can produce a nice selection of articles and decent discussion. HN is arguably higher quality, for now, but I don't have another good source for startup-related items. (Suggestions welcome.)
Just my personal preference, others may disagree and vote differently. It's just that lately it seems I'm sifting through an awful lot of general-interest articles, many of which I've already seen on reddit.
The more HN becomes a source for non-startup news, the more users we'll get who don't care about startups. Since HN doesn't have "subreddits," we'll end up with just another general news aggregator, gradually decreasing in quality like they all do.
I know that I've contributed to this, and I've decided to stop.
People like you, who come to HN for startup news
People like me, who come to HN for HN's take on news
The rift is causing more headaches for both sides as time goes on (and it's not going to go away on its own), so it's reasonable to split HN into two subsites: Hacker News and Startup News.