Because the writers engage each other and writers at competing sites more than the smartest readers. Fishbowls tend to attract both groupies and trolls. The former are prone to celebrity worship and the latter by the prospect of schadenfreude.

"Gawker" is a great name for this whole approach to media.

If you recognize that quality commenting is a DIFFERENT and complementary skill to quality writing, tou won't make the mistake of working with a class hierarchy mental model where writers outrank commenters. Instead you'll cultivate them. Authority in blogging is situational. In a healthy blog, writers outrank commenters on some topics, and are outranked by them on others.

Just encouraging adulation from groupies and not feeding trolls is a strategy for gradually hollowing out the commenter community. If you encourage and engage quality commenters and don't feed EITHER groupies or trolls, over time, you'll build a strong comments section. Both groupies and trolls will get bored and move on.

Unfortunately this is a very slow growth strategy. Gawker-blogging (the style, which Techcruch also uses, not the specific site) can grow traffic 10x as fast. Which strategy you choose should ideally depend on how long you intend the site to exist.

More broadly both groupies and trolls tend to be somewhat lonely, isolated types who derive a lot of their social sustenance through online interactions with strangers. Great commenters are in it to develop richer and more diverse relationships beyond what they already have.