alt Hacker News> signups are often still rejected with no explanation given
That's pretty odd. When I google this, there are claims that this is to prevent spam accounts. Obviously, if you choose not to take them at their word, one can read all sorts of odd reasons into it.
That exchange might be believable if bots didn't exist on X, but they absolutely do, hence the search for other explanations.
Considering who owns it, it'd be weird if they didn't want a carefully curated echo chamber of sanitized thought.