[jeroenhd]

I think grouping different communities together would be a great feature, but you'd still have to find someone to curate and collect those groups, and host them somewhere.

"Why are all of these communities there" is a bit of a weird question, in my opinion. Why is there a news.ycombinator.com when Reddit already existed? Why was reddit founded when Digg was around? Why aren't don't Outlook and Gmail combine forces into one? Why would anyone want to go to stackoverflow when expertsexchange and hundreds of forums already existed?

Every server has its own rules, that's why. Some are more tolerant of abusive behaviour under the banner of free speech, some will use bans liberally to maintain their community. Some servers ban NSFW posts, others don't care. Some servers block some other servers for abuse, others don't. Some communities are fine with meme posts mixed into serious posts, others want to stay on topic.

Even on a centralised platform such as Reddit there have always been duplicate subreddits because of people who disagreed with another subreddit's rules or moderation practices.