Sometimes it’s just that certain subreddits got brigaded a lot by users of another subreddit, so it was reasonable to proactively ban people as they were much more likely to do more harm than good. Usually, if that was the reason, you could ask to be let in via modmail.

Reddit never produced the brigade detection and prevention tools they were promising a decade ago, so if there was a big enough problem, the only options were some pretty blunt instruments.