Dan Abramov (React core team, formerly of Meta) talks about this in a Twitter thread: https://twitter.com/dan_abramov/status/1654688751342301184

In short, it's because Vercel invested engineer-years into piloting RSCs as part of a production framework and making Next the guinea pig. Next wasn't actually the first framework to try RSCs - Hydrogen (Shopify) and Gatsby both tried early versions of it and provided feedback, but Next is the first to implement it to the point that it can be considered stable.

The long-term goal is that RSCs aren't a Next-only concept - for example the Remix team said that they wouldn't implement them until they became more stable (https://twitter.com/ryanflorence/status/1678416472010260480), but are working on it now (https://twitter.com/ryanflorence/status/1697253955376722092). But the React team has always historically favoured trying new features in production to iterate on the API (this happened a lot internally at Meta), and this was an opportunity to do that.

Also worth noting that after experimenting with RSCs in Hydrogen, Shopify acquired Remix and is now building Hydrogen using Remix.

Also worth noting that Remix is working on RSC support right now.