At a guess:

- No "bad history" from submitter.

- No detected "obvious slop" signs

- Relatively (near zero) few comments during first hour, during which time it received steady, unclustered, unique upvotes.

- No actual mod took a look and weighted it either way

HN algo weights against rapid fire comment trees (sign of "controversy / chat" rather than thoughtful content (sort of)), obvious bot activity, upvotes from sketchy sources, etc - other than that submissions are pretty much bound for front page if they get a rate of organic votes.