It isn't just about being personal (though that's particularly bad). It's about not tossing in pejoratives and putdowns. The HN guidelines are clear about this:

"When disagreeing, please reply to the argument instead of calling names. 'That is idiotic; 1 + 1 is 2, not 3' can be shortened to '1 + 1 is 2, not 3."

We don't want name-calling (in this sense) because it leads to dumber, nastier internet discussion.

It's also particularly easy to avoid. If you take out its first sentence, the GP comment loses nothing but hostility and becomes a far better HN contribution.

https://news.ycombinator.com/newsguidelines.html