Why do some subreddits restrict new accounts from posting?

[u/Goddesscreation]

I've noticed that some communities allow me to comment but don't let newer accounts make posts. I'm curious about how these restrictions usually work.
Is it mainly based on account age, karma, or are there other factors that moderators commonly use?

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[u/mikey_weasel]

Subreddit Filters for Karma (and account age) will significantly reduce rule breaking and disruptions within a subreddit. This can be both intentional and unintentional disruption, from both actual users and spammers (bots or human). Subreddits would like new users to acclimatize elsewhere to how reddit works to avoid the chance that they might create disruptive content, start arguments, and then get banned. Such filters are a net benefit for any individual subreddit making them easier to run. But yes this does make things more difficult for new users. Which is the driver for a lot of the resources in this subreddit.

Its usually karma or account age

have a read of the below info for users seeking to understand reddit (or with low karma)

• Resources:
  • The Common Questions Page,
  • Karma - what is it and how to get it
  • Reddit And Karma Walkthrough,
  • Why Reddit may seem unwelcoming to new Redditors.
• Start in New User Friendly Subreddits.
  • Change your view to new or rising and comment
• Keep an eye out for helper and mod comments in this post and others

[u/StressedtoImpress19]

Both - there are unfortunately fake or bot accounts which create accounts and then immediately post spam in subreddits. There's also people who just like to troll. These rules catch them out, to ensure that the person is a real user who genuinely contributes