Because it is a marketing funnel for ycombinator.

then why call it 'hacker news' colloquially?

HN, originally called Startup News and later renamed to Hacker News, is a news board hosted by Y Combinator. Hence news.ycombinator.com. There's no hard rule saying that the official name of something has to be 1:1 match to its domain. And sure, YC could move HN to a separate domain, but why bother? Everyone's used to news.ycombinator.com.

Colloquially? "Hacker News" is its formal name.

It was also originally Startup News, and changed names about (iirc) 12 months later.