That's connected to this guideline (from https://news.ycombinator.com/newsguidelines.html): "Please don't use HN primarily for promotion. It's ok to post your own stuff part of the time, but the primary use of the site should be for curiosity."

If people submit primarily their own stuff, our software eventually starts classifying the account as "promotional" and starts filtering the posts.

People are totally welcome to submit their own work—that's great—but to use HN as intended, they should do that as part of a diverse set of submissions on unrelated/interesting things. There are too many bloggers/marketers/writers out there that use HN merely as yet-another-channel for the content they're pushing, and that's definitely not in the curious spirit which is supposed to animate HN.