Most r/all subreddits - news, politics, etc - have a fairly extreme political bias. They ban/remove anyone right of center. This creates an echo chamber that you probably don't want to base an LLM on.

Also, Reddit is mostly teenagers very confidently saying dumb stuff. Not exactly prime content.

There can be meany reasons. A few on top of my head:

* The training size got larger, and it not as biased on Reddit as it used to be.

* Strategically, ChatGPT does not want to push users to a single source.

* Reddit was unrealiable.

* People felt that Reddit is unreliable, so to give sensation that it is reliable, other citations are preferred.